Closed-loop individual cylinder A/F ratio balancing

ABSTRACT

The individual cylinder air to fuel ratio of an internal combustion engine varies due to the fact that the intake manifold cannot distribute airflow into the individual cylinders evenly. This feature of the present invention utilizes minimum timing for best torque MBT timing criterion provided by the ionization signals or by the in-cylinder pressure signals to balance the air to fuel ratios of the individual cylinders. The control method of the present invention comprises: calculating a mean timing coefficient, calculating a timing coefficient error, integrating the minimum timing for best torque timing coefficient error, calculating a raw fuel trim coefficient, resealing the raw fuel trim coefficient, updating a feedforward look-up table based upon the current engine operating conditions, and calculating a final fueling command.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application Ser.Nos. 60/423,163, filed Nov. 1, 2002, and 60/467,660, filed May 2, 2003,the entire disclosure of these applications being considered part of thedisclosure of this application and hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to internal combustion engine ignitionsystems. More particularly, it relates to a method of balancing air tofuel ratios of individual cylinders in an automobile's engine.

2. Discussion

The prior art includes a variety of conventional methods for detectingand using ionization current in a combustion chamber of an internalcombustion engine. However, each of the various conventional systemssuffers from a great variety of deficiencies. For example, prior artionization current detection circuits are generally too slow andgenerate a current signal with low signal-to-noise ratio.

Typically, the primary coil of an ignition system is charged close to adesired amount of energy as a function of engine operational conditionssuch as the local mixture air to fuel ratio, pressure, temperature, andengine gas recirculation concentration. Since the breakdown voltage andspark duration at the discharge moment can be different from cycle tocycle, it is desirable to monitor some of these parameters. Section Adiscloses how these parameters are monitored.

It is desirable to run an automobile internal combustion engine at itsminimum timing for best torque spark timing, if possible, for improvedfuel economy. Due to the lack of a combustion feedback control system inthe prior art, the ignition timing is controlled in open loop based uponan minimum timing for best torque timing table based upon the enginemapping data. The disadvantage of this approach is that it requires along calibration process and the minimum timing for best torque timingcontrol system is sensitive to changes in system parameters.

SUMMARY OF THE INVENTION

In view of the above, the described features of the present inventiongenerally relate to one or more improved systems, methods and/orapparatuses for detecting and/or using an ionization current in thecombustion chamber of an internal combustion engine.

In one embodiment, the present invention is a method of balancingindividual cylinder air to fuel ratios, comprising the step of inputtingat least one timing coefficient.

In another preferred embodiment, the step of inputting at least onetiming coefficient further comprises the steps of calculating a meantiming coefficient, calculating a timing coefficient error, integratingthe timing coefficient error, calculating a raw fuel trim coefficient,resealing the raw fuel trim coefficient, updating a feedforward look-uptable based upon the current engine operating conditions, andcalculating a final fueling command.

In a further preferred embodiment, the mean timing coefficient is a meanminimum timing for best torque timing coefficient, and the timingcoefficient error is a minimum timing for best torque timing coefficienterror.

In another preferred embodiment, the present invention is a method ofbalancing individual cylinder air to fuel ratios, comprising the stepsof calculating timing minimum timing for best torque information of thecylinder, and controlling the cylinder's air to fuel ratio using theminimum timing for best torque timing information.

In a further preferred embodiment, the timing information is calculatedusing an in-cylinder pressure signal.

In another preferred embodiment, the timing information is calculatedusing an ionization signal.

In a further preferred embodiment, the present invention comprises anair to fuel ratio control system, including an ionization detectioncircuit, a controller operably connected to the ionization detectioncircuit, memory, and software stored in the memory, wherein the softwarecomprises instructions which control a cylinder's air to fuel ratiousing minimum timing for best torque timing information.

Further scope of applicability of the present invention will becomeapparent from the following detailed description, claims, and drawings.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given here below, the appended claims, and theaccompanying drawings in which:

FIG. 1 illustrates an ionization feedback and control system;

FIG. 2 is a graph of an ionization signal;

FIG. 3 is a graph that compares the secondary signals and the ionizationsignals;

FIG. 4 is a graph of an ionization signal when the plug is fouled andthe insulator is overheated;

FIG. 5 illustrates the effect of pre-ignition on an ionization signal;

FIG. 6 is a diagnostics flowchart of the steps taken in the presentembodiment of a method of monitoring ignition efficiency;

FIG. 7 is a flowchart of the steps taken in the present embodiment todiagnose the ignition using the ionization signal;

FIG. 8 is an electrical schematic of a circuit for measuring ionizationcurrent in a combustion chamber of an internal combustion engine;

FIG. 9 a is a graph of the control signal V_(IN) from the PCM to theIGBT versus time.

FIG. 9 b is a graph of the current flow I_(PW) through the primarywinding of the coil versus time;

FIG. 9 c illustrates an output voltage signal Vout resulting from anormal combustion event.

FIG. 10 a is a block diagram of the ignition diagnostics and feedbackcontrol of the present invention;

FIG. 10 b is a block diagram of the ignition diagnostics and feedbackcontrol of the present invention containing the features of eachsubsystem;

FIG. 11 is a graph of an ionization current signal that is multiplexedwith charge feedback signal;

FIG. 12 is a drawing of the ignition diagnostics subsystem of thepresent invention;

FIG. 13 is a logic block diagram of the system architecture of theignition diagnostics and feedback control system;

FIG. 14 illustrates an Air/Fuel ratio sweep vs. crank angle;

FIG. 15 is a plot of a 300 cycle average ionization at wide openthrottle condition Air/Fuel ratio sweep at MBT for Lambda=1.2, 1.1, 1.0,0.95, 0.9, 0.85, 0.8

FIG. 16 illustrates an A/F ratio sweep at WOT;

FIG. 17 illustrates a 3000 rpm SA=20 BTDC Air/Fuel ratio sweep at WOT

FIG. 18 illustrates the A/F ratio perturbation of the present invention;

FIG. 19 illustrates the A/F ratio optimization of the present invention;

FIG. 20 illustrates the real-time WOT A/F ratio optimization method ofthe present invention;

FIG. 21 is a flowchart of the real-time WOT A/F ratio optimizationmethod of the present invention;

FIG. 22 is a logic block diagram of the real-time WOT A/F ratiooptimization controller of the present invention;

FIG. 23 illustrates the closed loop retard spark control usingionization current feedback apparatus of the present invention;

FIG. 24 is a flowchart of the steps taken in the present embodiment indeciding whether to advance or retard the ignition timing;

FIG. 25 a illustrates the closed loop cold start control method when thepartial burn index and the misfire index are inactive.

FIG. 25 b illustrates the closed loop cold start control method when thepartial burn index is active and the misfire index is inactive.

FIG. 25 c illustrates the closed loop cold start control method when themisfire index active.

FIG. 26 is a logic block diagram of the adaptive learning manager.

FIG. 27 are plots of three cases of ionization waveforms;

FIG. 28 is a block diagram of the multi-criteria MBT timing estimationmethod of the present invention;

FIG. 29 illustrates a logic block diagram of the present invention;

FIG. 30 is a flowchart of the steps taken by the multi-criteria MBTtiming estimation method and apparatus of the present invention;

FIG. 31 is a logic block diagram of an individual cylinder MBT timingcontroller;

FIG. 32 is a flowchart of steps taken by the PI controller of thepresent invention;

FIG. 33 is a logic block diagram of the closed loop MBT timing PIcontroller of the present invention;

FIG. 34 is a logic block diagram of the closed loop knock spark limitmanagement of the present invention;

FIG. 35 is a flowchart of the steps taken by the present inventionduring closed loop control when the engine is knock limited;

FIG. 36 is a logic block diagram of the closed loop retard timing limitmanagement of the present invention;

FIG. 37 is a flowchart of the steps taken by the present inventionduring closed loop control when the engine is misfire limited;

FIG. 38 is a logic block diagram of the average MBT timing control ofthe present invention;

FIG. 39 is a logic block diagram of the mixed MBT timing control of thepresent invention;

FIG. 40 is a plot of the relationship between A/F ratio and MBT sparktiming;

FIG. 41 illustrates the relationship between A/F ratio and MBT sparktiming for the individual cylinders of a 2.0 L, four cylinder engine;

FIG. 42 is a plot of the linear relationship between A/F ratio and MBTtiming information for the individual cylinders of a 2.0 L, fourcylinder engine;

FIG. 43 is a logic block diagram of the closed loop individual cylinderA/F ratio balancing control method of the present invention;

FIG. 44 is a flowchart of the closed loop individual cylinder A/F ratiobalancing control method of the present invention;

FIG. 45 illustrates the look-up table which comprises feedforward fueltrim coefficient vectors FT_(FDD);

FIG. 46 is a logic block diagram of the air to fuel ratio control systemof the present invention;

FIG. 47 is a plot of IMEP COP vs. MBT timing as a function of EGR rate;

FIG. 48 is a plot of the knock limited EGR rate of the presentinvention;

FIG. 49 is a logic block diagram of the closed loop EGR rate control ofthe present invention;

FIG. 50 is a flowchart of steps taken by the closed loop EGR ratecontroller of the present invention;

FIG. 51 a is a flowchart of steps taken by the logic blocks of FIG. 49;

FIG. 51 b is a flowchart of steps taken by the logic blocks of FIG. 49;

FIG. 52 is a graph of mass fraction burned and its first and secondderivatives;

FIG. 53 is a graph of net pressure its derivatives versus crank angle;

FIG. 54 is a graph of torque change with spark timing;

FIG. 55 is a graph of net pressure acceleration change with sparktiming;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention detects an ionization signal in an enginecombustion chamber from an ionization detection circuit. The system andassociated subsystems described herein use the detected ionizationsignal to monitor ignition parameters, diagnose and improve engineperformance, detect cylinder ID, control air-to-fuel ratio, controlspark retard timing, control minimum timing for best torque timing, andcontrol exhaust gas recirculation, in addition to other featuresdisclosed in the following embodiments. For clarity, it is noted thatmany of the details concerning the method and apparatus for usingestimated minimum timing for best torque timing criteria generated fromeither (or both) an ionization signal and an in-cylinder pressure signaland ignition diagnostics (knock, partial-burn, and misfire) to controlengine ignition timing according to the present invention are describedin Section G of this detailed description.

This detailed description includes a number of inventive featuresgenerally related to the detection and/or use of an ionization current.The features may be used alone or in combination with other describedfeatures. While one or more of the features are the subject of thepending claims, other features not encompassed by the appended claimsmay be covered by the claims in one or more separate applications filedon even date by or on behalf of the assignee of the present application.

For clarity, each of the features is described in separate sections ofthis detailed description. Section A discloses the use of an ionizationsignal from an ionization detection circuit to monitor ignitionparameters, such as primary charge timing (or time), primary chargeduration, ignition or spark timing, and ignition or spark duration forfuture “smart” ignition system control.

Section B discloses a circuit for measuring ionization current in acombustion chamber of an internal combustion engine in this circuit, theignition current and the ionization current flow in the same directionthrough the secondary winding of the ignition coil and the circuitdetects an ionization signal by applying a bias voltage between a sparkplug gap. Notwithstanding the described preferred circuit, those skilledin the art will appreciate that many of the features of the inventionmay be implemented through other ionization detection circuits ormethodologies without departing from the scope of the appended claims.

Section C discloses an ignition diagnostics and a feedback controlsystem based upon detected ionization current in an individual cylinder.The system is divided into two subsystems, the ignition diagnosticssubsystem and the ignition feedback control subsystem, both of whichfunction to improve fuel economy and reduce emission enginecalibrations.

Section D discloses the use of an ionization signal to optimize the airto fuel ratio of a combustion mixture when an engine is operated at wideopen throttle. The ionization signal is used to detect the air to fuelratio that yields the highest torque at wide open throttle. At the sametime, a closed loop controller is used to regulate the air to fuel ratiowhen the engine is operated at the wide open throttle.

Section E discloses using a closed loop spark timing controller tocontrol the spark retard timing during an engine cold start to retardthe engine spark timing as much as possible without engine misfire andwith minimum partial burn. The increased exhaust temperature heats upthe catalyst quickly which, as a result, reduces hydrocarbon emissions.

Section F discloses a method to determine engine minimum spark timingfor best torque timing at current operational conditions using a sparkplug ionization signal. It is a multi-criteria minimum timing for besttorque timing estimation method which utilizes a combination of themaximum flame acceleration location, the 50% burn location, and thesecond peak location to determine engine minimum timing for best torquetiming.

Section G discloses a subsystem comprising a closed loop controllerwhich uses estimated minimum timing for best torque timing criteriagenerated from either (or both) an ionization signal and an in-cylinderpressure signal and ignition diagnostics (knock, partial-burn, andmisfire) to control engine ignition timing. Three different embodimentsof the closed loop minimum timing for best torque timing controlarchitecture are disclosed. They are differentiated by whether theminimum timing for best torque timing is controlled cylinder-by-cylinderor globally. The first embodiment controls the engine minimum timing forbest torque spark timing of each cylinder individually. That is, theminimum timing for best torque, knock, and misfire information of agiven cylinder is used to control that cylinder's minimum timing forbest torque timing. The second embodiment uses an averaged approach. Thethird embodiment uses a mixed approach.

Section H uses the linear relationship between air to fuel ratio versusminimum timing for best torque criteria to balance the air to fuel ratiofor individual cylinders. In a preferred embodiment, a closed loopcontroller is used to adjust (or trim) the fuel of individual cylinderssuch that all cylinders have the same minimum timing for best torquetiming criterion.

Section I uses the ionization signal and closed loop control of theexhaust gas re-circulation to provide the engine with either the minimumtiming for best torque timing or knock limited timing to yield themaximum fuel economy benefits associated with exhaust gasre-circulation.

Section J uses the maximum acceleration rate of the net pressureincrease resulting from the combustion in a cylinder to control sparktiming.

Section A: Ignition Diagnosis Using Ionization Signal

This feature utilizes the ionization signal from an ionization detectioncircuit to monitor ignition parameters, such as primary charge timing(or time), primary charge duration, ignition or spark timing, andignition or spark duration for future “smart” ignition system control.In addition, the ionization signal is also used to detect spark plugcarbon fouling, insulator overheating, pre-ignition, as well as a failedionization circuit or ignition coil.

The performance of an engine is heavily dependent upon the performanceof its ignition system, especially at low load and high EGR (exhaust gasrecirculation) conditions. Understanding how the ignition system behavesat various engine conditions is very beneficial to “smart” control ofthe ignition system. Typically, the primary coil of an ignition systemis charged close to a desired amount of energy as a function of engineoperational conditions such as the local mixture A/F (air to fuel)ratio, pressure, temperature, and EGR concentration. The actual chargedenergy of the primary coil and discharged energy of the secondary coilare unknown. This leads to an ignition system that is not robust toparts-to-parts variation, engine aging, engine operational environmentalcharges, etc. To improve the ignition system robustness, a “smart”ignition system that can change its charged energy to match thedischarged energy is desirable. Therefore, the secondary dischargeinformation is very important. Since the breakdown voltage and sparkduration at the discharge moment can be different from cycle to cycle,it is desirable to monitor some of these parameters.

This invention uses the spark plug ionization signal to monitor theprimary charge time (or primary charge timing 146) and primary chargeduration, and also the secondary discharge time and duration to lay thefoundation for “smart” control of the ignition system 110. In addition,this invention also includes using the ionization signal to detect sparkplug malfunction, such as carbon fouling or insulator overheating 197,pre-ignition 190, and a failed ionization circuit or/and ignition coil.

This feature of the invention is generally directed to a subsystem of anignition diagnostics and feedback control system using ionizationcurrent feedback. The relationship of this subsystem to the diagnosticsand control system is shown in FIG. 1 in the top box “Ignition systemdiagnostics”, 140, 150, 146, 160, 170, and 197, which comprises thefollowing ignition parameters: ignition duration 170, charge duration150, warning signal 197, primary charge time 146, ignition timing 160,and pre-charge 140. The four blocks of the ignition diagnostics andfeedback control system using ionization current feedback that aredirected to spark timing 1480 are the CL knock (advance) limit control1450, the closed-loop MBT spark control 1430, 1490 and 1495, CL misfire& partial burn (retard) limit control 1460 and the CL cold start retardlimit control 1000. There are two blocks directed to the fuel trimvector 975, the individual cylinder A/F ratio control 1300 and the WOTA/F ratio optimization 1900. There is one block directed to the desiredEGR rate 1630, the EGR rate optimization 1600. The three other blocksshown in FIG. 1 are an analog signal processing block ASP, an A/Dconversion block A/D and a parameter estimation block 1800. Theparameter estimation block is shown outputting knock 1404, MBT 1435 andmisfire 1414 signals. The input to the analog signal processing blockASP is an ionization current 100.

A typical ionization signal 100 versus crank angle is shown in FIG. 2.Note that the signal shown is a voltage that is proportional to thedetected ionization current. Comparing the secondary voltage 120 andcurrent waveforms 130 (FIG. 3), it is clear that the initial rise of theion signal before the sharp change at the ignition time is thepre-charge (or start of charge) of the primary coil 140. See FIG. 2.After the primary coil charge is completed, the signal goes down andrises almost vertically (i.e., a step rise) versus crank angle. Thebreakdown has occurred at the step's rising edge. Spark timing can bedetected based on this point. That is, the ignition or spark time occurswhen the ionization signal has a step rise. This is the ignition orspark time 160. The time difference between the first rise and thestepped rise is the primary charge duration 150. When the arc betweenthe spark gap dies out, the signal declines rapidly and the secondarycurrent 130 due to the spark reaches zero (see FIG. 3). The durationbetween the sharp stepped rising and the subsequent declining representsthe ignition duration 170. Therefore, based on the ionization signal,the primary charge time 146, the primary charge duration 150, theignition timing 160, and the ignition duration 170 can be detected.These parameters can be monitored for every cylinder of the engine foreach engine cycle.

When a spark plug is fouled, or the spark plug insulator is overheated,or the plug itself is temporarily contaminated by fuel spray, theinsulator of the spark plug serves as a conductor. At these conditions,the ionization signal baseline is no longer equal to the bias voltage105. Depending on how badly the plug is fouled and how overheated theinsulator is, the ionization baseline will be elevated 180 (FIG. 4) fromthe bias voltage 105. Meanwhile, part of the ignition energy will leakthrough the fouled plug or the insulator during the primary chargeperiod. Eventually the remaining energy is not enough to jump the sparkgap and a misfire will occur (196) (FIG. 6). For some cases, thebaseline can be so high that it reaches the limit of the ionizationsignal and the signal becomes of little use. Once the baseline iselevated to (or beyond) a certain threshold (e.g., an elevation ofapproximately 20% or 1 volt above the initial baseline), a warningsignal indicating plug fouling or overheating will be sent out 197 (FIG.6).

When pre-ignition occurs in the cylinder, the ionization signal 100 willdetect ions before the ignition happens (190), see FIG. 5 which showspre-ionization due to pre-ignition. One pre-ignition cycle could lead toan even earlier pre-ignition during the next cycle and damage theengine. It is desirable to control the engine to a cooler operatingcondition once the pre-ignition is detected.

In order to detect an open or a shorted ionization circuit, the biasvoltage (105) is sampled far away from the ignition and combustionevents (e.g., 180 degree after the top dead center). If the sampled biasvoltage is below a given threshold (such as 0.5 volt), an openionization circuit or a short to ground fault can be detected 198 (FIG.6); if the bias voltage is greater than a threshold (e.g., 4.5 volts),an ionization circuit that is shorted to the battery shall be declared199 (FIG. 6). The open or short circuit information can then be used todiagnose the condition of the ignition system (see FIGS. 6 and 7).

Section B: Circuit for Measuring Ionization Current

FIG. 8 is a basic electrical schematic of a circuit 10 for measuringionization in a combustion chamber of an internal combustion engine. Thecomponents and configuration of the circuit 10 are described first,followed by a description of the circuit operation.

First, with regard to the components and configuration of this feature,the circuit 10 includes an ignition coil 12 and an ignition or a sparkplug 14 disposed in a combustion chamber of an internal combustionengine. The ignition coil 12 includes a primary winding 16 and asecondary winding 18. The ignition plug 14 is connected in electricalseries between a first end of the secondary winding 18 and groundpotential. The electrical connections to a second end of the secondarywinding 18 are described further below. A first end of the primarywinding 16 is electrically connected to a positive electrode of abattery 20. A second end of the primary winding 16 is electricallyconnected to the collector terminal of an insulated gate bipolartransistor (IGBT) or other type of transistor or switch 22 and a firstend of a first resistor 24. The base terminal of the IGBT 22 receives acontrol signal, labeled V_(IN) in FIG. 8, from a powertrain controlmodule (PCM) not shown. Control signal V_(IN) gates IGBT 22 on and off.A second resistor 25 is electrically connected in series between theemitter terminal of the IGBT 22 and ground. A second end of the firstresistor 24 is electrically connected to the anode of a first diode 26.

The circuit 10 further includes a capacitor 28. A first end of thecapacitor 28 is electrically connected to the cathode of the first diode26 and a current mirror circuit 30. A second end of the capacitor 28 isgrounded. A first zener diode 32 is electrically connected across or, inother words, in parallel with the capacitor 28 with the cathode of thefirst zener diode 32 electrically connected to the first end of thecapacitor 28 and the anode of the first zener diode 32 electricallyconnected to ground.

The current mirror circuit 30 includes first and second pnp transistors34 and 36 respectively. The pnp transistors 34 and 36 are matchedtransistors. The emitter terminals of the pnp transistors 34 and 36 areelectrically connected to the first end of the capacitor 28. The baseterminals of the pnp transistors 34 and 36 are electrically connected toeach other as well as a first node 38. The collector terminal of thefirst pnp transistor 34 is also electrically connected to the first node38, whereby the collector terminal and the base terminal of the firstpnp transistor 34 are shorted. Thus, the first pnp transistor 34functions as a diode. A third resistor 40 is electrically connected inseries between the collector terminal of the second pnp transistor 36and ground.

A second diode 42 is also included in the circuit 10. The cathode of thesecond diode 42 is electrically connected to the first end of thecapacitor 28 and the emitter terminals of the first and second pnptransistors 34 and 36. The anode of the second diode 42 is electricallyconnected to the first node 38.

The circuit 10 also includes a fourth resistor 44. A first end of thefourth resistor 44 is electrically connected to the first node 38. Asecond end of the fourth resistor 44 is electrically connected thesecond end of the secondary winding 18 (opposite the ignition plug 14)and the cathode of a second zener diode 46. The anode of the secondzener diode 46 is grounded.

Referring now to FIGS. 8 and 9, the operation of the circuit 10 isdescribed. FIG. 9 a is a graph of the control signal V_(IN) from the PCMto the IGBT 22 versus time. FIG. 9 b is a graph of the current flowI_(PW) through the primary winding 16 of the ignition coil 12 versustime. FIG. 9 c is a graph of an output voltage signal from the circuit10 versus time. As mentioned above, the IGBT 22 receives the controlsignal V_(IN) from the PCM to control the timing of 1) the ignition orcombustion and 2) the charging of the capacitor 28. In this circuitconfiguration, the IGBT 22 is operated as a switch having an OFF, ornon-conducting state, and an ON, or conducting state.

Initially, at time=t₀, the capacitor 28 is not fully charged. Thecontrol signal V_(IN) from the PCM is LOW (see FIG. 9 a) therebyoperating the IGBT 22 in the OFF, or non-conducting, state. Primarywinding 16 sees an open circuit and, thus, no current flows through thewinding 16.

At time=t₁, the control signal V_(IN) from the PCM switches from LOW toHIGH (see FIG. 9 a) thereby operating the IGBT 22 in the ON, orconducting, state. Current from the battery 20 begins to flow throughthe primary winding 16 of the ignition coil 12, the conducting IGBT 22,and the second resistor 25 to ground. Any of a number of switches orswitching mechanisms can be used to connect current through the primarywinding 16. In a preferred embodiment IGBT 22 is used. Between time=t₂and time=t₂, the primary winding current I_(PW), (illustrated in FIG. 8with a dotted line) begins to rise. The time period between time=t₁ andtime=t₂ is approximately one millisecond which varies with the type ofignition coil used.

At time=t₂, the control signal V_(IN) from the PCM switches from HIGH toLOW (see FIG. 9 a) thereby operating the IGBT 22 in the OFF, ornon-conducting, state. As the IGBT 22 is switched OFF, flyback voltagefrom the primary winding 16 of the ignition coil 12 begins to quicklycharge the capacitor 28 up to the required bias voltage. Between time=t₂and time=t₃, the voltage at the first end of the secondary winding 18connected to the spark plug 14 rises to the voltage level at which theignition begins. The time period between time=t₂ and time=t₃ isapproximately ten microseconds. The first resistor 24 is used to limitthe charge current to the capacitor 28. The resistance value of thefirst resistor 24 is selected to ensure that the capacitor 28 is fullycharged when the flyback voltage is greater that the zener diode.

At time=t₃, an ignition voltage from the secondary winding 18 of theignition coil 12 is applied to the ignition plug 14 and ignition begins.Between time=t₃ and time=t₄, combustion of the air/fuel mixture beginsand an ignition current I_(IGN) (illustrated in FIG. 8 with a dash-dotline) flows through the second Zener diode 46, the secondary winding 18of the ignition coil 12, and the ignition plug 14 to ground. At time=t₄,the ignition is completed and the combustion of the air/fuel mixturecontinues.

At time=t₅, the combustion process continues and the charged capacitor28 applies a bias voltage across the electrodes of the ignition plug 14producing an ionization current I_(ION) due to the ions produced by thecombustion process which flows from the capacitor 28. The current mirrorcircuit 30 produces an isolated mirror current I_(MIRROR) identical tothe ionization current I_(ION). A bias current I_(BIAS) (illustrated inFIG. 8 with a phantom or long dash-short dash-short dash line) whichflows from the capacitor 28 to the second node 48 is equal to the sum ofthe ionization current I_(ION) and the isolated mirror currentI_(MIRROR) (i.e., I_(BIAS)=I_(ION)+I_(MIRROR)).

The ionization current I_(ION) (illustrated in FIG. 8 with a dashedline) flows from the second node 48 through the first pnp transistor 34,the first node 38, the fourth resistor 44, the secondary winding 18 ofthe ignition coil 12, and the ignition plug 14 to ground. In thismanner, the charged capacitor 28 is used as a power source to apply abias voltage, of approximately 80 volts, across the spark plug 14 togenerate the ionization current I_(ION). The bias voltage is applied tothe spark plug 14 through the secondary winding 18 and the fourthresistor 44. The secondary winding induction, the fourth resistor 44,and the effective capacitance of the ignition coil limit the ionizationcurrent bandwidth. Accordingly, the resistance value of the fourthresistor 44 is selected to maximize ionization signal bandwidth,optimize the frequency response, and also limit the ionization current.In one embodiment of the present invention, the fourth resistor 44 has aresistance value of 330 k ohms resulting in an ionization currentbandwidth of up to twenty kilohertz.

The current mirror circuit 30 is used to isolate the detected ionizationcurrent I_(ION) and the output circuit. The isolated mirror currentI_(MIRROR) (illustrated in FIG. 8 with a dash-dot-dot line) is equal toor, in other words, a mirror of the ionization current I_(ION). Theisolated mirror current I_(MIRROR) flows from the second node 48 throughthe second pnp transistor 36 and the third resistor 40 to ground. Toproduce a isolated mirror current signal I_(MIRROR) which is identicallyproportional to the ionization current I_(ION), the first and second pnptransistors 34 and 36 must be matched, i.e., have the identicalelectronic characteristics. One way to achieve such identicalcharacteristics is to use two transistors residing on the same piece ofsilicon. The isolated mirror current signal I_(MIRROR) is typically lessthan 300 microamps. The third resistor 40 converts the isolated mirrorcurrent signal I_(MIRROR) into a corresponding output voltage signalwhich is labeled as V_(OUT) in FIG. 8. The resistance value of the thirdresistor 40 is selected to adjust the magnitude of the output voltagesignal V_(OUT). The second diode 42 protects the mirror transistors 34and 36 by biasing on and providing a path to ground if the voltage atnode 38 crosses a threshold. A third transistor can also be used toprotect the mirror transistor.

FIG. 9 c illustrates an output voltage signal V_(OUT) resulting from anormal combustion event. The portion of the output voltage signalV_(OUT) from time=t₅ and beyond can be used as diagnostic informationregarding combustion performance. To determine the combustionperformance for the entire engine, the ionization current in one or morecombustion chambers of the engine can be measured by one or morecircuits 10 respectively.

In the present circuit 10, the ignition current I_(IGN) and theionization current I_(ION) flow in the same direction through thesecondary winding 18 of the ignition coil 12. As a result, theinitiation or, in other words, the flow of the ionization current aswell as the detection of the ionization current is quick. In the presentcircuit 10, the charged capacitor 28 operates as a power source. Thus,the circuit 10 is passive or, in other words, does not require adedicated power source. The charged capacitor 28 provides a relativelyhigh bias voltage from both ionization detection and the current mirrorcircuit 30. As a result, the magnitude of the mirrored, isolated currentsignal I_(MIRROR) is large and, thus, the signal-to-noise ratio is high.

Section C: Ignition Diagnosis and Combustion Feedback Control SystemUsing an Ionization Signal

The spark ignition (SI) engine combustion process is governed by thein-cylinder air to fuel (A/F) ratio, temperature and pressure, exhaustgas re-circulation (EGR) rate, ignition time and duration, and otherfactors. Engine emission and fuel economy are dependent on the engine'scombustion process. For homogeneous combustion engines, most often theengine air to fuel (A/F) ratio is controlled in a closed loop using afeedback signal either from a heated exhaust gas oxygen (HEGO) or from auniversal exhaust gas oxygen (UEGO) sensor. The exhaust gasre-circulation (EGR) rate is controlled with the help of a delta (Δ)pressure measurement. Due to the high cost of an in-cylinder pressuresensor, engine spark timing is controlled in an open loop and iscorrected using a knock detection result. As a result of open loopcontrol, the engine combustion process is sensitive to operationalconditions, engine-to-engine variation, engine aging, and other relatedfactors. This sensitivity results in a complicated calibration processdue to engine mapping and the calibration process of various sparktiming lookup tables, trims and adders, where the spark timing tablesare used to vary the ignition/spark timing as function of engine speedand load, and the trims and adders are used to compensate engineignition/spark timing when a special engine operational condition occurs(e.g., transient operation). The present invention proposes an ignitiondiagnostics and feedback control system using an ionization current as afeedback signal to improve ignition system robustness with respect toengine operational conditions, engine-to-engine variations, engineaging, and other related factors to reduce engine calibration needs.

Spark ignited engine systems in the prior art have several disadvantagesand drawbacks. For example, the ignition control process is open loopand the actual ignition time and duration are unknown. Further, thecommand ignition time is controlled in an open loop with lookup tablesas function of engine speed, load, etc., along with trims and adders tocompensate engine operational condition variations. Additionally, thelimitations imposed by using accelerometer-based engine knock detectionprevent the spark ignition engine from running at its knock limit whenneeded, leading to reduced fuel economy.

In contrast, features of the present invention include individualcylinder diagnostics features such as ignition system diagnostics:charge timing 146, charge duration 145, ignition timing 160, ignitionduration 170, plug fouling 197, pre-ignition 190, etc.; misfiredetection: misfire flag 414, partial burn flag 412, and other relatedfactors; knock detection: knock flag 404 and knock intensity 402; andMBT timing detection: robust multi-criteria MBT timing estimator 200.

Furthermore, features of the present invention also include controlfeatures such as closed loop cold start retard spark control usingionization feedback 1000; Closed loop MBT timing control usingionization feedback 1430, 1490, 1495; Closed-loop individual cylinderA/F ratio balancing 1300; Optimal wide open throttle air/fuel ratiocontrol 1900; and Exhaust gas control using a spark plug ionizationsignal 1600.

The present invention includes an ignition diagnostics and a feedbackcontrol system based upon detected ionization current in an individualcylinder 801. The system 801 is divided into two subsystems asillustrated in FIGS. 10 a and 10 b. The ignition diagnostics subsystem802 and the ignition feedback control subsystem 803 both function toimprove fuel economy and reduce emission engine calibrations.

A typical ionization signal is plotted in FIG. 11 which shows that thedetected ion current signal 100 can be divided into two sections, chargeignition 141 and post-charge ignition signals 143.

The architecture of ignition diagnostics subsystem 802 is shown in FIG.12 and includes four main features. First, the ignition systemdiagnostics provides the ignition primary coil charge timing 146, chargeduration 145, secondary coil discharge timing (or in other words,ignition/spark timing 160), ignition duration 170, fault ignition system(such as failed coil, failed spark plug, etc.) using the charge-ignitionportion 141 of the ion current signal 100. Also, the post ignitionionization current signal 143 is used to detect spark plug fouling 197.

Second, the misfire detection feature provides individual cylindermisfire information 1410 such as misfire and partial burn conditionsusing the post ignition ionization current signal 143 and the results ofignition system diagnostics. The resulting misfire detection is muchmore accurate than existing engine speed based misfire detectionsystems, especially at the engine deceleration condition where currentdetection systems fail to provide an accurate detection of misfire. Theexisting speed based misfire detection systems also have difficulty indetecting misfire for those engines with more than eight cylinders.

Third, the knock detection feature provides knock intensity 1402 andknock flag 1404 signals based upon a band-path filtered post-ignitionportion of the ionization signal 100. One advantage of using anionization signal 100 for knock detection is that it enables individualcylinder knock detection and also produces a cleaner knock signal whencompared to current accelerometer based knock detection techniques whichrequire intensive calibration due to engine valve noises.

Fourth, the robust multi-criteria MBT (Minimum timing for Best Torque)timing estimation apparatus and method 200 provides a compound indexbased upon the post-ignition ionization current signal 143 for anindividual cylinder. This index combines multiple MBT timing indexeswhich are calculated using information provided by the ionizationcurrent signal 100 for the 10% mass fraction burned, the 50% massfraction burn, and the peak cylinder pressure (PCP) locations forimproved estimation robustness. When the engine is not knock limited,the index can be used for closed loop control of engine ignition/sparktiming for improved fuel economy, reduced emissions, reducedcalibration, etc.

The system architecture of the ionization feedback control subsystem 803is shown in FIG. 13 and includes five controllers: (1) a closed loopcold start retard spark control using ionization feedback 1000; (2) aclosed loop MBT timing control using ionization feedback 1430, 1490,1495; (3) a closed-loop individual cylinder air to fuel ratio balancingcontrol system 133; (4) optimal wide open throttle air/fuel ratiocontrol 1900; and (5) exhaust gas control using a spark plug ionizationsignal 1600.

As to the closed loop cold start retard spark control using ionizationfeedback 1000, it is noted that 70% of the HC emission during an FTPcycle is produced during a cold start since the catalyst temperaturedoes not reach its operational point quickly. Various approaches havebeen developed in the prior art to heat up the catalyst quickly duringthe cold start. One technique involves retarding the spark timingsignificantly to raise the exhaust temperature so that the catalyst canbe heated up quickly. However, since retarding the spark timing islimited by partial burn and misfire, open loop calibration of retardspark timing for a cold start is done very conservatively due toengine-to-engine variations, engine aging, operational conditionvariations, etc. Under closed loop control of retard spark timing of thepresent invention, the engine retard timing limit is adjusted during acold start to reduce the conservativeness and heat up the catalyst morequickly, thereby reducing HC emissions during a cold start.

As to the closed loop MBT timing control using ionization feedback 1430,1490, 1495, when the spark timing of an internal combustion engine isneither knock limited, nor misfire/partial-burn limited, the engineoperates at its MBT spark timing for best fuel economy when emission issatisfactory. Existing MBT timing control disclosed in the prior art iscontrolled in an open loop based upon engine mapping data. This approachdoes not compensate for engine-to-engine variations, engine operationalconditions, aging of components, and other related factors.Consequently, many ignition/spark timing corrections used to compensatefor those various conditions, called adders or trims, have to be addedfor improved engine performance. The closed loop MBT spark timingcontrol strategy of the present invention adjusts the engine sparktiming when the engine 161 is neither knock or misfire limited toprovide improved fuel economy. When the engine 161 is knock limited, theclosed loop control of the present invention adjusts the engineignition/spark timing so that the engine 161 runs at its knock limit,thereby providing improved fuel economy and high torque output.

The third controller is the closed-loop individual cylinder air to fuelratio balancing control system 1300. In the prior art, the airflowpassage of an intake manifold for each individual cylinder is quitedifferent. Consequently, the charge air volume and flow pattern for eachindividual cylinder is different, even at steady state operatingconditions. Thus, even if the mean air to fuel (A/F) ratio of allcylinders remains at stoich, the air to fuel ratio of each individualcylinder can differ from stoich. In the present invention, the fuelinjected for each individual cylinder is adjusted to ensure that eachcylinder has the same air to fuel (A/F) ratio. The proposed individualcylinder air to fuel (A/F) ratio balancing method and apparatus of thepresent invention utilizes MBT timing estimation obtained during closedloop MBT timing control to adjust/trim the fuel metered for eachcylinder using a fuel multiplier for each individual cylinder. Inaddition, a closed loop controller using a HEGO or UEGO sensor keeps themean air to fuel (A/F) ratio at stoich. By using the fact that the MBTspark timing of a rich cylinder is relatively retarded compared to oneat a stoich air to fuel (A/F) ratio and the MBT spark timing of a leancylinder is relatively advanced compared to a stoich one, the individualcylinder fuel multiplier can be modified based upon detected MBT timingindex or the resulting MBT timing control to balance relative air tofuel (A/F) ratios between individual cylinders.

As noted above, the fourth controller is the optimal wide open throttleair/fuel ratio control 1900. Normally, the air to fuel (A/F) ratio atwide open throttle (WOT) is adjusted to maximize the engine torqueoutput. At this point, the engine 161 is operated at its minimumadvanced MBT timing. In the prior art, air to fuel (A/F) ratio at WOT isoptimized using an open loop calibration approach based on enginemapping data. The apparatus and method of the present invention adjuststhe air to fuel (A/F) ratio to minimize MBT spark advance when theengine 161 is running at WOT.

Finally, with the exhaust gas control using a spark plug ionizationsignal 1600, an ionization signal 100 is used to calculate a combustionstability index. The combustion stability index can be the combustionburn rate or a parameter related to burn duration. The combustionstability index is then used to control the exhaust gas re-circulation(EGR) rate to increase the exhaust gas re-circulation EGR dilution. TheEGR rate is increased when the combustion stability index is below athreshold. This enables the engine to run at an increased EGR rate whilemaintaining stable combustion. As a result, fuel economy is improved,while emissions are decreased.

Section D: Optimal Wide Open Throttle Air/Fuel Ratio Control

This feature of the present invention uses an ionization signal tooptimize the air to fuel ratio (AFR) of a combustion mixture when anengine is operated at wide open throttle (WOT). This yields the highestBMEP (Brake specific Mean Effective Pressure), or in other words,maximizes the engine's torque output with the best fuel economy. Inaddition, spark timing is also optimized.

Engines are typically operated at a stoichiometric air to fuel ratio AFR(which is approximately 14.7 to 1 for gasoline) to optimize catalyticconverter performance. Operating an engine at below stoichiometric (lessthan 14.7 to 1) results in operating the engine with a rich air to fuelratio AFR. In this instance, the fuel does not completely combust andthe catalytic converter can get clogged from the resulting emissions. Onthe other hand, operating an engine at above stoichiometric (greaterthan 14.7 to 1) results in operating the engine with a lean air to fuelratio AFR. In this instance, there is excess oxygen in the emissions.This causes the catalytic converter to operate at an elevatedtemperature, thereby limiting the conversion of nitrogen-oxygencompounds (“NOx”). More importantly, operating at this condition forlong durations may damage the catalyst converter.

Normally, oxygen sensors are used when controlling an air to fuel ratioAFR. The oxygen sensor measures the presence of oxygen in the air/fuelmixture. However, when operating an engine at wide open throttle WOT,the air/fuel mixture is outside the stoichiometric range, normally belowthe stoich. The air to fuel ratio AFR and the spark timing for enginesrunning at wide open throttle WOT conditions use extensive calibrationto produce the best torque output. Due to the non-stoich operation atwide open throttle WOT, the oxygen sensor (which generates either a richor a lean signal) can not be used as an indicator of air to fuel ratioAFR. Therefore, the air to fuel ratio AFR is not being controlled in aclosed loop under wide open throttle operation conditions.

The present invention 900 uses the ionization signal to detect the airto fuel ratio AFR that yields the highest torque or BMEP at wide openthrottle WOT. At the same time, a closed loop controller is used toregulate the air to fuel ratio AFR when the engine is operated at thewide open throttle WOT. In addition, the engine spark timing isoptimized at its minimum timing for the best torque (MBT) timing for thecorresponding conditions.

Optimal WOT A/F ratio detection involves optimizing the A/F ratio atwide open throttle WOT condition. When an engine is operating at wideopen throttle WOT condition, it is desired that the engine outputs thehighest BMEP (torque) to meet the torque demand. The highest BMEP ateach engine operating condition is not only a function of the sparktiming, but also a function of the air to fuel ratio AFR if the air tofuel ratio AFR is allowed to deviate from the stoich air to fuel ratio.When an appropriate spark timing is found for the operating condition,the highest BMEP is established through the most efficient combustion.The best combustion efficiency can be achieved when the combustiblemixture attains its fastest laminar flame speed. For most fuels, thefastest laminar flame speed usually occurs at an equivalence ratio φequal to 1.1 (where λ, defined as the inverse of φ, equals to 0.9). Theexcess-air factor, λ, is a factor which indicates the amount that an airto fuel ratio AFR is above or below a stoichiometric mixture. Thus, atλ=1.0, the air/fuel mixture is at stoichiometric. At λ=1.3, the air/fuelmixture is 130% of stoichiometric or 30% above stoichiometric.

Because an oxygen sensor (which provides either a rich or a lean switchsignal) is of minimal use when trying to sense the air to fuel ratio AFRfar away from the stoichiometric mixture, existing technology uses anopen loop control strategy and extensive calibration work when an engineoperates at wide open throttle WOT conditions. A sensor that can detectan efficient combustion condition is needed to control the air to fuelratio AFR at wide open throttle WOT conditions in a closed loop.

FIG. 11 is a typical ionization signal 100 detected from a spark plug105 inside the combustion chamber. It is a plot of the ionization signal100 during both charge ignition 141 and post charge ignition 143. Afterthe spark breakdown, a flame kernel is formed in the spark gap. Thefirst peak 162 of the ionization signal 100 is produced as a result ofthe initial flame formation. The chemical reaction caused by the flameformation increases the number of ions present in the engine cylinder.After the flame kernel is well established, the flame front graduallypropagates away from the gap and the ionization signal 100 graduallydeclines. Meanwhile, the flame front pushes both unburned and burnedgases in front of it and behind it and causes the local temperature inthe vicinity of the gap to increase along with the cylinder pressure.Since the mixture around the spark gap is the first part of the mixtureburned in the cylinder and is the first part of the burned mixture thatis compressed in the cylinder, the local temperature of the air/fuelmixture is always highest in the gap. As the flame propagates away, theionization signal 100 starts to increase again due to the elevatedtemperature. When the cylinder pressure reaches its peak, the gaptemperature also reaches its hottest point. Therefore, the second peak166 of the ionization signal 100 occurs as a result of the secondaryionization due to the high temperature.

U.S. Pat. No. 6,029,627 discloses that the first peak 162 reaches itshighest value when the excess-air factor λ is between 0.9 and 0.95. Inaddition, the second peak 166 reaches its highest value when λ is around1.1. At a light load condition, the first peak 162 most likely peaksaround λ=0.9 as shown in FIG. 14. However, when the engine load getshigher, the first peak 162 increases as λ increases beyond 0.9. Theincrease in the first peak 162 is due to increased carbon speciesdissociation caused by higher temperatures due to the increased loadcondition. In addition, the second peak 166 does not usually peak aroundλ=1.1 as described in U.S. Pat. No. 6,029,627, but instead occurs aroundλ=0.9.

Around λ=0.9, both flame speed and flame temperature are at theirhighest. The fastest flame speed indicates the most efficient combustionprocess. The appearance and placement of the second peak 166 depends ondifferent loads, sparks and air to fuel ratios and might not appear atall at some operating conditions. However, at wide open throttle WOTconditions, the second peak 166 will always show up when the mixture isricher than stoichiometric AFR.

Maximizing either the valley value 164 or the second peak value 166versus the air to fuel ratio AFR can be used as a criterion to find themost vigorous combustion condition. This condition usually occurs when λis between 0.9 and 0.925. From FIG. 15 and FIG. 17, it is clear thatthis criterion holds true when minimum timing for the best torque MBTtiming is used for each air to fuel ratio AFR condition. In FIG. 17, thesecond peak 166 is about 2.6 volts when λ is between 0.9 and 0.925 andthe valley 164 is about 1.3 volts. Both the second peak 166 and thevalley 164 are at maximum values. This criterion also holds true when afixed spark timing is used for 1500 rpm wide open throttle WOT conditionand 2000 rpm wide open throttle WOT condition as shown in FIG. 14 andFIG. 16 respectively.

To improve the robustness of the optimal air to fuel ratio AFR detectioncapability, the values for the valley 164 and the 2^(nd) peak 166 arecombined to estimate the optimal air to fuel ratio AFR during the wideopen throttle WOT operation.C _(AFR)=(V _(valley) +V _(2nd-PEAK))/2,  (Equation 1).V_(valley)+V_(2nd−PEAK) is plotted in FIG. 17. It reaches a maximumaround λ=0.9.

Real-time optimal A/F ratio control algorithm: Note that the air to fuelratio AFR index C_(AFR) for a specific air to fuel ratio AFR does notprovide information whether the air to fuel ratio AFR, that the engineis operated at, is optimal or not. A completed relationship of C_(AFR)and air to fuel ratio AFR is used to determine the preferred air to fuelratio AFR at wide open throttle WOT operation. However, thisrelationship is a function of many factors, such as engine-to-enginevariation, engine aging, and engine operational conditions (altitude,humility, etc.). Consequently, it is difficult to optimize air to fuelratio AFR for wide open throttle WOT conditions using off-lineoptimization.

This feature of the present invention optimizes the wide open throttleWOT air to fuel ratio AFR on-line using the relationship between C_(AFR)and air to fuel ratio AFR at WOT. Similar to a closed loop stoich air tofuel ratio AFR control system, an air to fuel ratio AFR perturbation (oroffset) is added to the desired mean air to fuel ratio AFR. See FIG. 18where Δ_(AFR) and T_(P) are the magnitude and period of the air to fuelratio AFR perturbation or air to fuel ratio AFR offset, respectively.The typical value of the perturbation magnitude Δ_(AFR) is 0.05, and thetypical perturbation period is between a quarter second and half asecond with a 50 percent duty cycle. An optimal WOT air to fuel ratioAFR control gradient parameter can be defined as:P _(AFR)=(C _(A/F)(H)−C _(A/F)(L))/Δ_(AFR)  (Equation 2)where air to fuel ratio AFR index C_(AFR)(H) corresponds to the maximumA/F ratio index obtained when the air to fuel ratio AFR is perturbed byadding Δ_(AFR), and C_(AFR)(L) corresponds to the minimum air to fuelratio AFR index obtained when the air to fuel ratio AFR is perturbed bysubtracting Δ_(AFR). For a typical case that the nominal λ is 0.925 withΔ_(AFR) equal to 0.05 when the engine is running at 3000 rpm with wideopen throttle, C_(A/F)(H) is 1.85 and C_(A/F)(L) 1.95. Since the air tofuel ratio AFR index C_(AFR) is a convex function of the air to fuelratio AFR (see both FIGS. 16 and 17), there are three possible ratiogradients for P_(AFR) (see FIG. 19):

-   -   P_(AFR)>0: The engine overall air to fuel ratio AFR with respect        to optimal AFR at WOT is rich;    -   P_(AFR)=0: The engine overall air to fuel ratio AFR is optimized        for best torque; and    -   P_(AFR)<0: The engine overall air to fuel ratio AFR with respect        to optimal AFR at WOT is lean.

In a preferred embodiment, the real-time control strategy adjusts theengine air to fuel ratio AFR based upon the air to fuel ratio AFRgradient parameter. During the wide open throttle WOT operation, theengine desired mean excess-air correction factor Δλ is updated using:Δλ_(DESIRED)(k+1)=Δλ_(DESIRED)(k)+α*P _(AFR)  (Equation 3),where α>0 is a calibratable constant coefficient for the real-timeoptimization algorithm. In this case, when the P_(AFR) is greater than 0(air to fuel ratio AFR is rich), a positive correction (αP_(AFR)>0) isadded to the desired mean excess-air correction factor (Δλ_(DESIRED)) byreducing the desired fuel quantity to increase the engine mean A/Fratio, thereby increasing the percentage of air. When P_(AFR) is lessthan 0 (air to fuel ratio AFR is lean), a negative correction(αP_(AFR)<0) is added to decrease the desired mean excess-air correctionfactor Δλ, thereby increasing the desired fuel quantity and decreasingthe percentage of air. When P_(AFR) is equal to 0, no adjustment isrequired.

FIG. 20 is a diagram of the WOT air to fuel ratio AFR control methoddiscussed above. Each step is number coded and the details are describedbelow. In step 910 the valley and 2^(nd) peak values are found. Moreparticularly, the valley value 164 and the 2^(nd) peak value 166 iscalculated using the ionization signal as discussed in “Optimal WOT A/Fratio detection,” where the definition of the valley 164 and the 2^(nd)peak 166 are found in FIGS. 14 and 16. This step is updated every firingevent.

In step 920, C_(AFR)(H) and C_(AFR)(L) are calculated using equation 1.As described in FIG. 18, a positive or negative perturbation is added tothe desired mean excess-air correction factor Δλ. When the positiveperturbation is added, C_(AFR)(H) is calculated, and when the negativeperturbation is added, C_(AFR)(L) is calculated. The mean values ofC_(AFR)(H) and C_(AFR)(L) of a perturbation period are used as output ofthis step. Therefore, this step runs every firing event, but outputevery perturbation period (Tp). For port injection engines, due to fueltransport delay, the calculation will be delayed until the transition iscompleted.

In step 930, the air to fuel ratio AFR control gradient P_(AFR) iscalculated. This step runs every air to fuel ratio AFR perturbationcycle. In order to make sure that the WOT air to fuel ratio AFR isoptimized at a given engine speed, this step calculates P_(AFR) when theengine speed variation is within a calibratable value.

In step 940, the updated desired air to fuel ratio correction factorΔλ_(DESIRED)(k+1) is calculated using Equation 2. This step runs everyair to fuel ratio AFR perturbation period. In the cases when the P_(AFR)is not calculated due to larger engine speed variation,Δλ_(DESIRED)(k+1) shall be set to zero.

In step 950 the feedforward air to fuel ratio λ_(FFD) is calculated. Thefeedforward air to fuel ratio AFR is based upon a lookup table that is afunction of engine speed 135 and other factors. This table provides anopen loop desired air to fuel ratio AFR for the engine system. Normally,this table is obtained through the engine calibration process. Theconventional calibration process to obtain the feedforward table is tomap the WOT engine output torque as a function of air to fuel ratio AFRat each given engine speed. Then, the feedforward table can be obtainedby selecting the AFR associated with the maximum WOT torque output atvarious engine speeds. For the control system with adaptive learningcapability, e.g., step 960, the feedforward table will be updated, usingthe calculated Δλ_(DESIRED)(k+1), to compensate for engine-to-enginevariations, engine aging, etc.

In step 960, the feedforward control FFD is updated. This step updatesthe feedforward control portion of the WOT air to fuel ratio AFR. Thestep calculates the difference between current feedforward output andthe final desired air to fuel ratio Δλ_(DESIRED)(k+1), and uses thedifference to update the feedforward table gradually. An engine speedsignal 135 is received from an engine speed sensor 136 located in theengine 161. The engine speed is used to as the input of the feedforwardlookup table. This step runs every perturbation period.

In step 970, the commanded fuel signal is calculated. This stepcalculates the commanded fuel signal based upon the desired air to fuelratio λ_(DESIRED)(k+1), where the λ_(DESIRED)(k+1) equals toΔλ_(DESIRED)(k+1)+λ_(FFD), and the current engine airflow rate {dot over(m)}_(AIR) 137 received from an air-mass flow sensor 138 located in theengine 161. The desired fuel flow rate {dot over (m)}_(FUEL)(k+1) equalsto current engine airflow rate {dot over (m)}_(AIR) divided by desiredair to fuel ratio λ_(DESIRED)(k+1). This step updates the engine fuelcommanded every engine combustion event or is executed at the same rateat which the fueling step runs.

It is a goal of the present control method to maintain the optimal airto fuel ratio AFR gradient P_(AFR) at zero. With the help of the convexproperty of C_(AFR) as a function of WOT air to fuel ratio AFR, thisgradient approach method shall converge with a proper calibrated α.

In a preferred embodiment, the steps (or instructions) in FIGS. 20 and21 are stored in software or firmware 107 located in memory 111 (seeFIG. 22) 900. The steps are executed by a controller 121. The memory 111can be located on the controller or separate from the controller 121.The memory 111 can be RAM, ROM or one of many other forms of memorymeans. The controller 121 can be a processor, a microprocessor or one ofmany other forms of digital or analog processing means. In a preferredembodiment, the controller is engine control unit ECU 121.

The ECU 121 receives an ionization signal 100 from an ionizationdetection circuit 10. The ECU 121 executes the instructions 107 storedin memory 111 to determine a desired air to fuel ratio AFR. It thenoutputs the desired fuel command 975 to some form of fuel controlmechanism such as a fuel injector 151 located on the engine 161.

Section E: Closed Loop Cold Start Retard Spark Control Using IonizationFeedback

Air pollution from automobile exhaust is caused in part by hydrocarbon(HC) emission. A catalyst converter is used in an internal combustionengine to reduce these pollutants by converting the harmful materials toharmless materials. Because the catalyst converter does not operate whenthe temperature of the catalyst is below its operational point, around70% of the hydrocarbon (HC) emission during the FTP (Federal TestProcedure) cycle is produced during the cold start when the catalysttemperature is below its operational point. Various approaches have beendeveloped to heat up the catalyst (or achieve catalyst light-off)quickly during the cold start. One way involves retarding (or delaying)the spark time (or ignition timing) to raise the exhaust temperature. Asa result, the catalyst heats up quickly during the cold start, reducingHC emission. Since spark retard is limited by partial burn and misfire,open loop calibration of a retard spark for a cold start is performedslowly and conservatively. The conservativeness of the open loopcalibration is mainly due to engine-to-engine variations, engine aging,operational condition variations, etc.

The retard spark control method of the present invention uses a closedloop spark timing controller to adjust the engine retard limit during acold start. The goal is to run an engine at its retard limit withoutpartial burn and misfire during a cold start to reduce the time neededto quickly heat up the catalyst. Therefore, use of a retard spark forfast heating of a catalyst during a cold start is maximized andcold-start hydrocarbon (HC) emission during a cold start is reduced.

The feature of the present invention described in Section E comprises asubsystem of the ignition diagnostics and feedback control systemdisclosed in FIG. 13 which uses ionization feedback current to raise thecatalyst temperature quickly. The relationship of the subsystem to thediagnostics and control system is shown in FIG. 13, where the cold startretard spark control is marked as logic block 1000. The method comprisesusing a closed loop to control the spark retard timing during an enginecold start to retard the engine spark timing as much as possible withoutengine misfire and with minimum partial burn. The increased exhausttemperature heats up the catalyst quickly which, as a result, reduces HCemissions.

The closed loop cold start retard spark control system using ionizationfeedback loop 1010 of the present invention 1000 is illustrated in FIG.23. A cold start enable flag 1020 (or command or signal) is used toenable (or activate) the closed loop control. The enable flag 1020 isgenerated when the catalyst temperature (measured or estimated) crossesa threshold (1015). The typical range of the threshold is about 400degree Celsius.

Inputs to the closed loop 1010 can include some or all of the signalsdiscussed below which include a partial burn index 1030, a misfire index1040, engine speed (RPM) 135, engine load 1060, and engine coolanttemperature 1070. In addition, the loop inputs are not limited to thesesignals, but can, in other embodiments, receive additional inputs. Thepartial burn index signal 1030 is obtained during a parameter estimationprocess for misfire detection. The misfire index signal 1040 is obtainedduring a misfire detection calculation by integrating the ion currentduring the combustion process and/or the peak value of the ionizationcurrent during the combustion. A threshold is used for the misfirecalculation. The current engine speed 135 measured in RPM (RevolutionsPer Minute). The engine load 1060 is calculated as a percentage ofmaximum load, fueling or the Indicated Mean Effective Pressure (IMEP).The engine coolant temperature signal 1070 is a conditioned enginecoolant temperature signal.

The signal output from the closed loop is the cold start spark signal1080 which is an ignition timing signal which will fire a spark plug ata Crank-angle After Top Dead Center (CATDC). Normally, the spark plug ina cylinder will be fired at its MBT timing normally located before thetop dead center (TDC). However, the ignition timing can be retarded (ordelayed) causing the spark plug to fire at retard timing (e.g., aftertop dead center) to increase the exhaust gas temperature, therebyheating the catalyst quickly.

The closed loop retard spark control using ionization current feedback1010 consists of four major components or functions (see FIG. 23) whichinclude an error and gain generator 1100, a proportional and integration(PI) control processing block 1200, a default spark timing processor1210, and an adaptive learning apparatus 1220. They are listed belowwith detailed descriptions.

FIG. 23 illustrates four major components of the closed loop controller1010 of the present invention. The first is the error and gain generator1100. The partial burn index signal 1030 and the misfire index signal1040 are input to the error and gain generator 1100. In a preferredembodiment, the error and gain generator 1100 can be a processor,microprocessor or any form of processing means. The misfire index iscalculated in Section A of this application by using the ionizationcurrent signal, and the partial burn index can be calculated using theinformation calculated during the misfire detection process, such asarea integration of the ionization current over the combustion windowand the peak value over the combustion window. By setting properthresholds higher than the misfire ones, partial burn index can beobtained by comparing the thresholds and calculated value. The error andgain generation generator 1100 outputs two signals, CL_error 1090 andCL_gain 1095, where CL_gain consists of both proportional andintegration gains. Signals CL_error 1090 and CL_gain 1095 can be any ofthree output values depending on the states of the partial burn indexand the misfire index, “negative one”, “one” and a “calibratablepositive value”:

The partial burn index signal 1030 and the misfire index signal 1040 areinput to the error and gain generator 1100 (see FIG. 24). Check thesignals' states 1115. If both the partial burn index 1030 and themisfire index 1040 are inactive 1120, then a closed loop error signalCL_error 1090 is set to “one”, the proportional gain of a closed loopgain signal CL_gain 1095 is set to “zero,” while the integration gain ofCL_gain is set to a calibratable positive value 1130. The typical rangeof the calibration positive value is between 0.01 and 2. In response tothese inputs, the control output 1205 of the proportional andintegration (PI) control processing block 1200 will move the closed loopcontrol's output signal 1080 in the retard direction 1140, therebydelaying the ignition timing. Thus, the firing of the spark plug isdelayed for that cylinder, causing the spark plug to fire at a moreretard crank-angle than the previous ignition event. FIG. 25 a shows howthe control method works for this case.

If the partial burn index is active and the misfire index is inactive1150, then CL_error 1090 is set to negative “one,” the proportional gainof CL_gain 1095 is set to “zero,” while the integration gain of CL_gain1095 is set to a calibratable positive value 1160 at a similar range tocase 1. In response to these inputs, the control output 1205 of theproportional and integration (PI) control processing block 1200 willmove the spark timing output signal 1080 in the spark advance direction1170. Thus, the firing of the spark plug is advanced for that cylindercausing the spark plug to fire before the previous spark timing. FIG. 25b shows how the control method works for this case.

If the misfire index signal 1040 is active 1180, then CL_error 1090 isset to negative “one,” the proportional gain of CL_gain 1095 is set to“zero,” while the integration gain of CL_gain 1095 is set to acalibratable positive value greater than cases 1 and 2. The upper boundof the calibratable positive value can reach 4 to avoid misfire in nextcombustion event. In response to these inputs, the proportional andintegration (PI) control processing block will add a calibratablenegative value (or offset, e.g., −5 degree) to the PI integrator causingits control output 1205 to move the spark timing output signal (orignition timing signal) 1080 in the spark advance direction to avoidmisfire and return to either case 1 or case 2 (1195). When a misfireoccurs, the PI integrator is reset by adding a calibratable sparkadvance (a negative value) to the existing integrator register toeliminate the misfire quickly. FIG. 25 c shows how the control methodworks for this case.

Therefore, the general method of the present invention comprises runningthe engine spark time at its retard limit. That is, to run the engine ata maximum allowable retard time without misfire and with minimum partialburn of the air-fuel (A/F) mixture. Thus: 1) when the engine is not atpartial burn, the spark timing moves in the retard direction at acertain rate 1140, such as quarter crank degree per combustion event; 2)when the spark timing is at partial burn, the spark timing moves in theadvance direction at a certain rate 1170 similar to case 1; and 3) whena misfire occurs, a correction will be added to the PI integrator tomove the spark timing in the advanced direction quickly to avoid furthermisfire 1195.

The second major component is the proportional and integration (PI)control processing block 1200. In a preferred embodiment, only theintegration portion of the PI controller 1200 or integration controller1200 is used for closed loop control of the retard spark during a coldstart. Both the integration gain CL_gain 1095 and the integration errorCL_error signals 1090 are provided by the error and gain generator 1100to the PI controller 1200.

The third major component is the default spark timing processor 1210.Default (or reference) spark timing is stored in a lookup table 1213which is a function of engine speed 135, engine load 1060, enginecoolant temperature 1070 and other factors. It can be obtained from theengine calibration process. The lookup table 1213 can be stored inmemory located in the default spark timing processor 1210 or in adiscrete memory chip. Due to the adaptive learning feature of thisprocessor or controller 1210, the default spark timing table 1213 ismodified by input 1225 (the adaptive learning output signal) from theadaptive learning apparatus 1220 so that the default (or reference)spark timing signal 1215 is compensated by the default spark timingprocessor 1210 or timing processor 1210 to account for engine-to-enginevariation, engine aging, and other related factors. The output from thedefault timing apparatus 1210 is the reference signal 1215 which issummed with the output 1205 of the PI controller 1200 by summer 1230 toproduce the cold start spark signal 1080.

The fourth major component is the adaptive learning apparatus 1220. Theadaptive learning apparatus 1220 compares (in comparator 1224) thecurrent cold start spark timing output signal ST_(CURRENT) 1080 (orcurrent spark timing signal 1080 or current ignition timing correctionsignal 1080) with a default cold start spark timing signal ST_(DEFAULT)1221 generated from a lookup table 1223 based upon the engine's currentoperating conditions (135, 1060, 1070) which serve as inputs to theadaptive learning block 1220. The lookup table 1223 can be stored inmemory located in a processor 1222 or in a discrete memory chip. Whenthe engine operational condition is close to a neighborhood of any meshpoint of default spark timing lookup table 1223, the spark timing valueST_(TABLE)(OLD) of the lookup table 1223 at that specific pointST_(TABLE) will be replaced by ST_(TABLE)(NEW) using the followingformula:ST _(TABLE)(NEW)=ST _(TABLE)(OLD)=β*(ST _(CURRENT) −ST _(DEFAULT)) 1226,where β is a calibratable positive coefficient with a typical value of0.02. In order to limit the capability of the adaptive algorithm forsafety and other reasons, the maximum allowed change of ST_(TABLE)(NEW)1226 from the default calibration can not be greater than a calibratablecrank degree. If the change of calculated ST_(TABLE)(NEW) 1226 exceedsthe limit, the boundary value will be used as ST_(TABLE)(NEW) 1226.

In one preferred embodiment, the adaptive learning apparatus 1220comprises a processor 1222, a comparator 1224, and software stored inmemory comprising instructions 1227 (which can be the same memory 1223that table 1223 is stored in or in different memory). For the defaultspark timing lookup table, see FIG. 26. The operating conditions includeengine speed (rpm) 135, engine load 1060 and coolant temperature 1070.The adaptive learning apparatus 1220 together with the default sparktiming processor 1210 constitute the feedback portion 1217 or feedbackcontroller 1217 of the closed loop 1010.

Section F: Robust Multi-Criteria MBT Timing Estimation Using IonizationSignal

It is a goal of an ignition system of an internal combustion engine totime the ignition/spark so that the engine produces its maximum braketorque with a given air to fuel mixture. This ignition/spark timing isreferred to as the minimum timing for best torque or MBT timing. Themean brake torque of an internal combustion engine is a function of manyfactors such as air to fuel ratio, ignition/spark timing, intake airtemperature, engine coolant temperature, etc. By fixing all the factorsthat affect the mean brake torque of an internal combustion engine, theengine mean brake torque is a convex function of ignition/spark timingwhen the ignition/spark timing varies within a certain range, where MBTtiming corresponds to the peak location of the convex function. If thespark timing is retarded or advanced relative to the MBT timing, themean brake output torque is not maximized. Hence, running an internalcombustion engine at its MBT timing provides the best fuel economy.Therefore, it is desirable to find criteria which can be used to producea reliable estimate of MBT timing for closed loop control of engineignition/spark timing. This invention proposes a method to determineengine MBT timing at current operational conditions using a spark plugionization signal.

Different from the cylinder pressure signal that exhibits a relativelystable pressure curve throughout engine operating conditions, thewaveform shape of a spark plug ionization signal can change with varyingloads, speeds, spark timings, air to fuel A/F ratios, exhaust gasre-circulation EGR rates, etc. Searching for the ionization post flamepeak that is supposed to be lined up with the peak pressure location isnot always a reliable MBT timing criteria due to the disappearance ofthis peak at low loads, retarded spark timing, lean A/F ratios, orhigher EGR rates. The present invention solves this problem byestablishing a robust multi-criteria MBT timing estimation methodutilizing different ionization signal waveforms that are generated underdifferent engine operating conditions.

The spark plug ionization signal 100 is a measure of the localcombustion mixture conductivity between the spark plug electrodes duringthe combustion process. This signal 100 is influenced not only by thecomplex chemical reactions that occur during combustion, but also by thelocal temperature and turbulence flow at the spark gap during theprocess. The ionization signal 100 is typically less stable than thecylinder pressure signal that is a measure of the global pressurechanges in the cylinder.

The MBT timing control strategies appearing in the prior art arepredominantly based on post flame peak detection. The post flame peakdetection usually lines up with the peak pressure location. It has beenrecognized that the MBT timing occurs when pressure location is around15° After Top Dead Center (ATDC). By advancing or delaying the sparktiming until the second peak of the ionization signal peaks around 15°ATDC, it is assumed that the MBT timing is found.

Unfortunately, the second peak of the ionization signal 100 does notalways appear in the ionization signal 100 waveform at all engineoperating conditions. At light loads, lean mixture, or high EGR rates,the second peak can be difficult to identify. Under these circumstances,it is almost impossible to find the MBT timing using the 2^(nd) peaklocation of the ionization signal 100.

The present invention establishes multiple MBT timing criteria toincrease the reliability and robustness of MBT timing estimation basedupon spark plug ionization signal 100 waveforms. Therefore, the presentmethod optimizes ignition timing by inferring from the ionization signalwhere the combustion event is placed in the cycle that corresponds tothe MBT timing.

FIG. 11 shows a typical ionization signal 100 versus crank angle.Different from a pressure signal waveform, an ionization signal 100waveform actually illustrates more details of the combustion process.For example, the ionization signal 100 waveform shows when a flamekernel is formed and propagates away from the gap, when the combustionis accelerated intensively, when the combustion reaches its peak burningspeed, and when the combustion ends. An ionization signal 100 usuallyconsists of two peaks. The first peak 162 of the ionization signal 100represents the flame kernel growth and development, and the second peak166 is caused by re-ionization due to the temperature increase resultingfrom pressure increase in the cylinder.

The combustion process of an internal combustion engine 161 is usuallydescribed using the mass fraction burn versus crank angle. Through massfraction burn, we can find when the combustion has its peak burningacceleration and peak burning velocity. Locating these events at aspecific crank angle will allow us to obtain the most efficientcombustion process. In other words, we can find MBT timing through theseevents. The inflexion point 163 right after the first peak 162 can becorrelated to the maximum acceleration point of the cylinder pressuresignal. This point is usually between 10% to 15% mass fraction burned(see FIG. 27, point 163 of case 1). The inflexion point 165 right beforethe second peak 166 of the ionization signal 100 correlates well withthe maximum heat release point of the cylinder pressure signal and islocated around 50% mass fraction burned (see FIG. 27, point 165 of case1). In addition, the second peak 166 is related to or correlates to thepeak pressure location of the pressure signal (see FIG. 27, point 166 ofcase 1).

At MBT timing, the maximum flame acceleration point is located at TopDead Center TDC. It has been established that the 50% mass fraction burnis located around 8–10° After Top Dead Center (ATDC) and the peakpressure location is around 15° ATDC when a combustion process starts atMBT timing. Combining all three individual MBT timing criterion orcriteria into one produces increased reliability and robustness of theMBT timing prediction.

FIG. 27, cases 1, 2 and 3, illustrate three waveforms that theionization signal takes at various engine operating conditions (case1—1500 rpm, 2.62 bar BMEP, EGR=0%; case 2—1500 rpm, 2.62 bar BMEP,EGR=15%; case 3—3000 rpm, WOT, cylinder #3): Case 1 illustrates a normalwaveform where both peaks 162, 166 are present in the waveform. In Case2, the second peak 166 does not show up due to the relatively lowtemperature resulting from the high EGR, a lean mixture or a low loadcondition, or from a combination of these factors. In Case 3, the firstpeak 162 merges with the ignition signal due to the longer crank angleignition duration resulting from a relatively constant spark duration athigh engine speed.

Locations 162–166 in FIG. 28 are defined as follows: 162, the first peaklocation of the ionization signal; 163, the maximum flame accelerationlocation (close to or correlated to Top Dead Center (TDC) at MBTtiming); 164, the valley location of the ionization signal; 165, themaximum heat release location (correlated to 50% burn location and closeto 8–10% After Top Dead Center (ATDC) at MBT timing); and 166, thesecond peak location (correlated to peak cylinder pressure location andclose to 15–17° After Top Dead Center (ATDC) at MBT timing).

In a preferred embodiment, the present invention uses MBT timingestimation criterion which is a combination of the maximum flameacceleration location 163, the 50% burn location 165, and the secondpeak location 166 which are shown in cases 1 through 3 of FIG. 27.

When the ionization signal 100 waveform takes on the waveform of case 1,all three MBT timing criteria will be used due to their availability.That is,L _(MBT)=(L ₁₆₃+(L ₁₆₅ −L _(50% BURN))+(L ₁₆₆ −L _(PCP)))/3,  (Equation1)where L_(MBT) is the estimated MBT timing location, L₁₆₃ is the maximumflame acceleration location, L₁₆₅ is the maximum heat release location,L_(50%BURN) is the 50% burn location of the pressure signal when theengine is running at MBT timing, L₁₆₆ is the second peak location, andL_(PCP) is the Peak Cylinder Pressure location when the engine isrunning at MBT timing. L_(50%BURN) and L_(PCP) are typically locatedaround 8–10° and 15–17° ATDC respectively. Since MBT timing forL_(50%BURN) and L_(PCP) varies as a function of engine operationalconditions, in a preferred embodiment a lookup table containing valuesof L_(50%BURN) and L_(PCP) can be used when calculating the desired MBTlocation for different operating conditions.

For case 2, the only available MBT criterion is location 163. Therefore,equation 1 reduces to:L _(MBT) =L ₁₆₃,  (Equation 2)where L_(MBT) is the estimated MBT timing location.

For case 3, locations L₁₆₅ and L₁₆₆ are available. The MBT timingcalculation utilizes both L₁₆₅ and L₁₆₆ calculate the estimated timinglocation as follows:L _(MBT)=((L ₁₆₅ −L _(50%BURN))+(L ₁₆₆ −L _(PCP)))/2,  (Equation 3)As with Case 1, both L_(50%BURN) and L_(PCP) can be selected to beconstant values (i.e., (8–10°) and (15–17°), respectively) or one canuse the outputs of lookup tables to reflect variations due to engineoperational conditions. The lookup tables 113 can be stored in memory111. Any form of memory such as RAM, ROM or even an analog memorystorage such as magnetic tape can be used. The data in the lookup tablecan be accessed by a processor, microprocessor, controller, enginecontrol unit, or any of a number of processing or controlling means 122.

FIG. 28 is a block diagram of the robust multi-criteria MBT timingestimation method of the present invention. FIG. 29 illustrates a logicblock diagram of the present invention 1800. The engine control unit ECU121 receives the ionization signal 100 from an ionization detection unit10 (Step 1801). (See FIG. 30). Next, the processor 122 in the ECUexecutes software or firmware 107 stored in memory (which can be thesame memory 111 in which the lookup table 113 is stored or differentmemory). The software 107 comprises instructions to determine which casethe ionization signal 100 waveform fits, i.e. case 1, 2 or 3 (1810). Ifionization signal 100 fits case 1 (1815), calculate locations L₁₆₃, L₁₆₅and L₁₆₆ (1817). Next, the software 105 calculates MBT timing byexecuting equation 1,L _(MBT)=(L ₁₆₃+(L ₁₆₅ −L _(50%BURN))+(L ₁₆₆ −L _(PCP)))/3(1820).If ionization signal 100 fits case 2 (1825), calculate location L₁₆₃(1827). Next, the software 105 calculates MBT timing by executingequation 2, L_(MBT)=L₁₆₃ (1830). If ionization signal 100 fits case 3(1835), calculate locations L₁₆₅ and L₁₆₆ (1837). Next, the software 105calculates MBT timing by executing equation 3,L _(MBT)=((L ₁₆₅ −L _(50%BURN))+(L ₁₆₆ −L _(PCP)))/2(1840).

The ECU 121 calculates an ignition timing control signal Vjn, using aclosed loop MBT timing controller (e.g., the one described in SectionG), and outputs it to a driver circuit 75 (1850). The driver circuit 75charges the ignition coil 12 which current to flow between the sparkplug 14 electrodes. The air to fuel (A/F) mixture between the electrodesconducts heavily, dumping the energy stored in the ignition coil 12 inthe spark plug 14 gap. The sudden release of energy stored in the coil12 ignites the air to fuel (A/F) mixture in the cylinder.

Section G: Closed Loop MBT Timing Control Using Ionization Feedback

This feature of the invention comprises a method and apparatus tocontrol the engine (minimum timing for the best torque (MBT) sparktiming in a closed loop using either ionization or pressure feedback.Both an ionization signal and an in-cylinder pressure signal can be usedfor calculating and estimating engine MBT timing criterion (or criteria)for each individual cylinder, where this criterion provides a relativemeasure of how far away the current engine spark timing is from MBTtiming. See Sections F, “Robust Multi-Criteria MBT Timing EstimationUsing Ionization Signal,” and J, “The Determination of MBT TimingThrough the Net Pressure Acceleration of the Combustion Process”. Whenthe engine is neither knock spark limited (i.e., the MBT spark is moreadvanced than knock limited spark timing), nor misfire/partial-burnlimited (i.e., the desired spark is more retarded (or delayed) than themisfire/partial-burn limited spark timing), the engine is operated in aclosed loop MBT timing control mode using MBT timing criterion feedback.

On the other hand, when the engine is knock limited, a knock limitmanager maintains operation of the engine at its non-audible knock limitusing a closed loop knock limit control. When the engine ismisfire/partial-burn limited (e.g., during the cold start, it isdesirable to run the engine at its retard limit to heat up the catalystquickly, see Section E), the misfire limit manager maintains the engineat its misfire/partial-burn limit.

It is desirable to run an automobile internal combustion engine at itsMBT spark timing, if possible, for improved fuel economy. Due to thelack of a combustion feedback control system in the prior art, theignition timing is controlled in open loop based upon an MBT timingtable based upon the engine mapping data. One disadvantage of thisapproach is that it requires a long calibration process and the MBTtiming control system is sensitive to changes in system parameters. Inother words, the open loop MBT timing control is not able to compensatethe MBT spark timing changes due to engine-to-engine variations, engineaging, and engine operational condition variations (altitude,temperature, etc.). In addition, the long calibration process extendsthe development duration and increases costs. An open loop ignitiontiming control limits its calibration to a conservative calibration sothat the engine cannot be operated at its physical limits (e.g., knocklimit). This, in turn, reduces fuel economy.

The present invention uses a closed loop MBT timing control, with thehelp of knock and misfire/partial burn limit management, to improve therobustness of an open loop ignition timing control. This, in turn,reduces engine system calibrations, and improves engine fuel economy.

The present invention comprises a subsystem of an ignition diagnosticsand feedback control system using ionization feedback. The relationshipof this subsystem to the diagnostics and control system is shown in FIG.13 and the logic blocks are labeled 1450, (1430, 1490, 1495), and 1460.This subsystem comprises a closed loop controller which uses estimatedMBT timing criteria generated from either (or both) an ionization signal100 and an in-cylinder pressure signal and ignition diagnostics (knock,partial-burn, and misfire) to control engine ignition timing. When theengine is not knock limited, it operates at its MBT timing for the bestfuel economy. When the engine is knock limited, the engine runs at itsnon-audible knock limit for the best torque output. When the engine ismisfire/partial-burn limited, the engine is maintained at itsmisfire/partial-burn limit.

Three different embodiments of the closed loop MBT timing controlarchitecture are discussed below: a) a cylinder-by-cylinder approach, b)an average approach, and c) a mixed approach. They are differentiated bywhether the MBT timing is controlled cylinder-by-cylinder or globally.The first embodiment controls the engine MBT spark timing of eachcylinder individually. The MBT, knock, and misfire information of agiven cylinder is used to control that cylinder's MBT timing. The secondembodiment uses an averaged approach. To be more specific, all cylindersare controlled using a single MBT timing control parameter. The thirdembodiment uses a mixed approach. That is, the engine misfire and knockare controlled individually, while the MBT timing is controlledglobally. The following is a detailed description of each embodiment.

In the cylinder-by-cylinder approach, the MBT timing criterion 1435, theknock information 1400 and the misfire information 1410 of eachindividual cylinder are calculated separately. Furthermore, the engine'sindividual MBT timing controller 1430 controls the ignition timing (seeFIG. 31). The cylinder-by-cylinder closed loop controller 1430 runsevery ignition event. The output of this closed loop controller is therecommended MBT timing signal 1480 for individual cylinders. The inputsto this closed loop controller 1430 are listed below:

The individual cylinder MBT criterion 1435 or individual cylinder MBTtiming criterion 1435 is calculated from an ionization signal 100 orin-cylinder pressure signal generated using a parameter estimationmethod (see Sections F, “Robust Multi-Criteria MBT Timing EstimationUsing Ionization Signal” and J, “The Determination of MBT Timing Throughthe Net Pressure Acceleration of the Combustion Process”) for theionization case. This parameter discloses whether the current enginespark is before or behind the MBT spark timing for that individualcylinder.

Individual cylinder knock information 1400 consists of a knock intensityparameter 1402 and a knock flag 1404. The knock intensity 1402 indicateshow severe the knock is and the knock flag 1404 indicates if an audibleknock exists or not. Note that both the knock intensity 1402 and theknock flag 1404 can be obtained from either ionization 100 orin-cylinder pressure signals.

Individual cylinder misfire information 1410 consists of bothpartial-burn 1412 and misfire flags 1414. Again, both partial-burn 1412and misfire flags 1414 can be obtained from ionization current 100 orin-cylinder pressure signals.

The cylinder-by-cylinder closed loop MBT timing controller 1430 of thepresent invention consists of three major subsystems: 1) a closed loopMBT timing proportional and integral (PI) controller 1440, 2) a knockspark advance limit manager 1450, and 3) a misfire spark retard limitmanager 1460. The MBT criterion 1435 is compared with the MBT referencesignal 1437 (1500), and the resulting error 1438 is input to the PIcontroller 1440 (1510) (See FIG. 32). The output 1442 of the PIcontroller 1440 provides the desired MBT timing if the engine 161 isneither knock limited, nor misfire limited. The saturation manager 1470outputs the ignition/spark timing signal 1480 used to control theignition timing for that particular cylinder. In a preferred embodiment,the saturation manager 1470 can prevent output windup.

The knock limit manager 1450 provides a knock limit signal 1452 thatprovides the maximum spark advance allowed at the current engine 161operational conditions. For example, when the engine 161 is not knocklimited, the knock manager 1450 provides a spark advance limit signal1452 associated with the engine's 161 physical configuration andcalibration. When the engine 161 is knock limited, the knock manager1450 provides a spark advance limit signal 1452 that makes it possiblefor the engine 161 to run at its knock limit, or in other words, allowsthe engine 161 to run with light or non-audible knock.

Similarly, the misfire limit manager 1460 provides a misfire limitsignal 1462. For example, when the engine 161 is without partial-burn ormisfire, it provides the engine 161 with a physical retard spark limitsignal 1462 associated with the engine's 161 physical configuration andcalibration. When the engine 161 spark timing is misfire limited, itprovides a retard limit signal 1462 to allow the engine 161 to operateat its partial-burn/misfire limit. The retard spark limit signal 1462 isinput to the saturation manager 1470.

When the engine 161 is either knock or misfire limited (that is, thesaturation manager 1470 is active), the corresponding knock 1452 orretard limit 1462 information is forwarded by the saturation manager1470 and is used to reset the PI integrator (see integration reset logicbelow) to avoid integration rewinding problems.

The PI controller 1440 of FIG. 31 (see FIG. 33 for a detailedconfiguration) consists of proportional 1441 and integral controller1444, a feedforward controller 1446 with adaptive learning capability1447, and an integration reset logic manager 1448 to prevent integrationcontrol overflow and rewinding problems. The functionality of theselogic devices is described below:

The feedforward controller 1446 is designed to modify open loop MBTtiming over the given engine operation map. In addition, an adaptivelearning manager 1447 is used to compensate for engine-to-enginevariation, engine aging, and operation environmental variation etc. Thefeedforward controller 1446 outputs feedforward output 1449.

The proportional 1441 and integral controllers 1444 output aproportional control output 443 and an integral controller output 1445respectively. The proportional control output 443 is produced bymultiplying the MBT error input 1438 by the proportional gain (1520)producing a proportional error signal 1443. The typical value for theproportional gain is around 0.2. The integral controller output 1445 isproduced by multiplying the integrated MBT error 1438 by the integralgain (1530) producing an integrated error signal 1445. The typical valuefor the integral gain is around 0.1. The integrated error signal can bereset when the engine is knock or misfire limited (see below). Theproportional error signal 1443, the integrated error signal 1445, andthe feedforward output 1449 are summed to produce timing signal 1442.

The integration reset manager 1448 is a logic device 1448 that becomesactive when the engine 161 is knock limited or misfire limited. Thereset integrated error signal is calculated in such a way that the sum1442 of the feedforward 1449, proportional 1443, and integral controller1445 outputs are limited by either the knock 1450 or the misfire limit1460 managers. That is, if the engine is knock or misfire limited, thenthe integral error signal is reset (1540) such that the final outputstays right at either the knock limit or misfire/partial-burn limit.

The second major subsystem, the knock spark advance limit manager 1450controls the closed loop knock limit. It includes a PI controller 51441,51444, 51446, 51448, 51447, a knock error and gain generator 1454 and asaturation manager 1470.

Only the integration portion of the PI controller (which comprisesblocks 51441, 51444, 51446, 51448, 51447 in the knock manager 1450) isused for closed loop knock limit control 1450 (see FIG. 34) since theproportional gain is set to zero at all times. The integration gain andthe error used by the PI controller 51441, 51444, 51446, 51448, 51447are provided by the knock error and gain generator 1454 (1552) (see FIG.35). The integral reset logic device 51448 is used to reset the integralgain and integrator controller 51444 output 51445 to avoid overflow andrewinding when the output is saturated (1540).

The closed loop knock spark advance limit manager 1450 (see FIG. 34 fora detailed configuration) consists of a knock error and gain generator1454 which is operably connected to the PI controller 51441, 51444,51446, 51448, 51447 disclosed in the knock manager 1450 in FIG. 34.

The knock error and gain generator 1454 is a logic block or device inwhich both the knock intensity 1402 and the knock flag 1404, calculatedin a preferred embodiment by using the ionization current signal 100,are used as inputs. The generator 1454 outputs two signals, “Error” 1455and “Gain” 1459, where “Gain” 1459 consists of both proportional andintegration gains. Both the “Error” 1455 and “Gain” 1459 outputs aregenerated using the knock intensity 1402 and knock flag 1404 signals andare divided into three states: a) no knock, b) inaudible knock, and c)audible knock.

The no knock state occurs when the knock flag signal 1404 is inactiveand the knock intensity 1402 is below the no knock threshold. In thiscase, if the knock flag signal 1404 is inactive and the knock intensity1402 is below the no knock threshold (1554), the “Error” output 1455 isset to one. In addition, the proportional gain of the “Gain” output 1459is set to zero, while the integration gain is set to a positive value(1556) such as 0.2, that may be calibrated. Thus, the proportionalcontrol output 51443 of the PI controller 51441, 51444, 51446, 51448,51447 is zero, while the integral controller output 51445 equals apositive value. The knock error and gain generator 1454 moves the closedloop timing output 51442 in the advance direction between the hardadvance lower limit 1456 and the hard advance upper limit 1458 since theintegration reset logic 51448 resets the integrator to limit the timingoutput within the timing boundary defined by 1456 and 1458 if the timingoutput 51442 is outside the boundary.

The inaudible knock state occurs when the knock flag signal 1404 isinactive and the knock intensity 1402 is beyond the knock threshold.This is the desired operational condition when the engine 161 is knocklimited. In this case, if the knock flag signal 1404 is inactive and theknock intensity 1402 is beyond the non-audible knock threshold (1560),the “Error” output 1455 is set to zero. In addition, the proportionalgain of the “Gain” output 1459 is set to zero, while the integrationgain remains at the calibrated value (1562) as in the no knock case.Thus, the proportional control output 51443 of the PI controller 51441,51444, 51446, 51448, 51447 is zero, while the integral controller output51445 remains at its previous positive value. This allows the timingadvance limit signal 1452 to remain unchanged.

The audible knock state occurs when the knock flag 1404 becomes active.In this case, if the knock flag 1404 is active (1565), the “Error”output 1455 is set to negative one. In addition, the proportional gainof the “Gain” output 1459 is set to zero, while the integration gain isset to a calibratable value (1567) such as 0.4. Plus, a calibratablenegative value is added to the integrator to move the spark timing inthe retard direction to avoid engine knock and to return to either caseb or case a, immediately.

The general method of the closed loop knock limit management is to allowthe engine to run its spark timing right at its advance limit (hardadvance upper limit 1458) or as close as possible. That is, when theengine 161 is knock limited, the engine 161 will run at its maximumadvance timing limit with a inaudible knock (i.e., case b, timing atknock limit DBTDC). When the engine 161 is not knock limited, sparktiming signal 1452 moves in an advanced direction at a certain rateuntil it reaches the hard limit. When the engine 161 runs right at itsinaudible knock limit, the spark timing limit signal 1452 remainsunchanged. And when the engine 161 runs with an audible knock, acorrection will be added to the knock PI integrator 51441, 51444, 51446,51448, 51447 to move the spark timing signal 1480 in the retarddirection quickly to avoid further engine knocking.

The feedforward knock spark limit controller 51446 and the adaptiveknock spark limit controller 51447 set a feedforward spark limit that isa function of engine speed 135 and engine load 1060. It can be obtainedthrough the engine calibration process. Due to the adaptive learningfeature of this controller 51447, the feedforward spark limit ismodified based upon the output of the adaptive learning method so thatthe feedforward spark limit is able to compensate for engine-to-enginevariation, engine aging and so on. The adaptive learning controller51447 compares the current spark limit with the feedforward timing limitsignal 51442 at the current engine operating conditions (such as enginespeed and load) to correct the feedforward timing limit 51442adaptively.

The third major subsystem, the closed loop misfire controller 1460 ormisfire retard limit manager 1460 controls closed loop misfire. Theclosed loop misfire controller 1460 (see FIG. 36 for a detailedconfiguration) consists of a misfire error and gain generator 1463.Here, the PI controller 61441, 61444, 61446, 61448, 61447 is used togenerate a spark advance limit signal. The integration gain and theerror used by the PI controller 61441, 61444, 61446, 61448, 61447 areprovided by the misfire error and gain generator 1463 (1572) (see FIG.37).

Only the integration portion of the PI controller 61441, 61444, 61446,61448, 61447 is used for closed loop control of the retard spark limit.Both the integration gain and error are provided by the error and gaingeneration block (or error and gain generator) 1463. When a misfireoccurs, the PI integrator 61444 is reset by adding an adjustable sparkadvance (a positive value) to the existing integrator register toquickly eliminate the misfire.

In the misfire error and gain generator block 1463, both thepartial-burn flag 1412 and the misfire flag 1414, which in a preferredembodiment, is calculated using the ionization current signal 100, areused as inputs. This block outputs signals, “Error,” 61455 and “Gain,”61459, where the “Gain” signal consists of both the proportional and theintegration gains. They can be divided into three states: d) bothpartial burn 1412 and misfire 1414 flags are inactive, e) partial burnflag 1412 is active, but the misfire flag 1414 is not active, and f)active misfire flag (or index) 1414.

In the case where both partial burn 1412 and misfire 1414 flags areinactive (1574), the “Error” output 61455 is set to a negative one, theproportional gain of the “Gain” output 61459 is set to zero, while theintegration gain of the “Gain” output 61459 is set to a positive value(1576) such as 0.2, that may be calibratal. Thus, the proportionalcontrol output 61443 of the PI controller 61441, 61444, 61446, 61448,61447 is zero, while the integral controller output 61445 decreases.This allows the closed loop control output 1462 to move in the retarddirection until it reaches the hard retard upper limit 1468. Note thatwhen ever output 1462 is not between the hard upper and lower limits(1468 and 1466), the integrator will be reset by the reset logic 61448such that the output stays within the range.

In the case where the partial burn flag 1412 is active, but the misfireflag 1414 is not active (1578), the “Error” 61455 output is set to one,and the proportional gain of the “Gain” output 61459 is set to zero,while the integration gain of the “Gain” output 61459 is set to a anadjustable positive number (1580) same as case d. Thus, the proportionalcontrol output 61443 of the PI controller 61441, 61444, 61446, 61448,61447 is zero, while the integral controller output 61445 is set to apositive value. This allows the spark timing 1462 to move in an advancedirection.

In the case where the misfire flag (or index) 1414 is active (1582), the“Error” output 61455 is set to one, and the proportional gain of the“Gain” output 61459 is set to zero, while the integration gain of the“Gain” output 61459 is set to an adjustable value greater than case e(1584) such as 0.4. Thus, the proportional control output 61443 of thePI controller 61441, 61444, 61446, 61448, 61447 is zero, while theintegral controller output 61445 moves in an advance direction. Anadjustable positive value is added to the PI integrator 61444 toimmediately move the closed loop control output signal 1462 in theadvanced direction to avoid misfire and to return to either case e orcase d, immediately.

The general method of the closed loop misfire spark limit control is toprovide the engine spark timing signal 1462 right at its retard limit.That is, to allow the engine to run at its maximum allowed retard time(i.e., maximum delay from the MBT timing for that cylinder) without amisfire and with minimum partial burn. When the engine 161 is not at thepartial burn state, the spark timing signal 1462 will move in the retarddirection at a certain rate determined by the integration gaincalibrated in case d. When the engine 161 is at partial burn, the sparktiming 1462 moves in the advance direction at a certain rate calibratedby the integration gain defined in case b. In the case where a misfireoccurs, a correction will be added to the PI integrator 61444 to movethe spark timing signal 1462 in the advance direction quickly to avoidfurther misfires.

The feedforward retard spark limit controller 61446 and the adaptiveretard spark limit learning controller 61447 set a feedforward retardspark limit that is a function of engine speed 135 and engine load 1060.It can be calculated during the engine's calibration process. Due to theadaptive learning feature of the controller 61447, the feedforward sparklimit is modified based upon the output of the adaptive learning methodso that the feedforward spark limit is able to compensate forengine-to-engine variations, engine aging, etc. The adaptive learningcircuit 61447 compares the current retard spark limit with the defaultlimit at the current engine operating conditions (such as engine speed135 and load 1060) to correct the feedforward retard spark limitadaptively.

As stated supra, the second embodiment of the MBT timing controlarchitecture uses an average approach. In this embodiment, the knockinformation 1400 and the misfire information 1410 of all the individualcylinders are used to calculate the worst case knock 1406 and worst casemisfire information 1416 which is then fed into the advance 1450 and theretard limit 1460 managers. A knock processor 1408 and a misfireprocessor 1418 perform the calculations. In addition, the latest engineMBT criterion 1435 is used to control the ignition timing, see FIG. 38.

The worst case knock information 1406 consists of both a worst caseknock flag 1407 and a worst case knock intensity 1409. The worst caseknock flag 1407 is set to active as long as one of the individualcylinder knock flags 1404 is active over one engine cycle. The worstcase knock intensity 1409 is equal to the maximum of all knockintensities 1402 for all the cylinders over one engine cycle.

Similar to the worst case knock information 1406, the worst case misfireinformation 1416 consists of both a worst case partial-burn flag 1417and a worst case misfire flag 1419. As long as one of the partial-burn1412 or misfire 1414 flags is active over one engine cycle, thecorresponding worst case partial-burn 1417 or misfire flag 1419 is setto active over one engine cycle.

The function of the MBT timing controller 1490 of the average approachembodiment is similar to the function of the controller used in thecylinder-by-cylinder method embodiment (compare FIG. 31 and FIG. 38). Inaddition, the average approach method uses only one PI controller 1440,one knock limit manager 1450 and one misfire limit manager 1460 togenerate one mean MBT ignition timing control signal 1480 which is usedto control the ignition for all cylinders. A difference between thisembodiment and the cylinder-by-cylinder embodiment is that the worstcase knock 406 and misfire 1416 information is used by the knock advancelimit 1450 and the misfire retard limit 1460 managers respectively (seeFIG. 38). In addition, current MBT criterion (or criteria) 1435 for thecurrent cylinder is input to the MBT PI controller 1440. The advantageof the average method is that only one PI controller 1440 is used forall cylinders which reduces the throughput requirement. However, sincethis method does not use individual cylinder knock and misfire limitmanagement, a more conservative knock and misfire control of eachcylinder occurs since one mean signal 1480 is used for all cylinders.

As stated supra, the third embodiment of the MBT timing controlarchitecture uses a mixed approach. In this embodiment, the individualknock 1400 and misfire information 1410 is used to calculate both knock1400 and misfire information 1410 for each cylinder. In addition, thecurrent knock 1400 and misfire information 1410 is input to the MBT PIcontroller 1440. Also, the current engine MBT criterion 1435 for thecurrent cylinder is used to control ignition timing, see FIG. 39.

The mixed MBT control method 1495 runs every combustion event. The knock1450 and misfire limit 1460 managers select the knock and misfire limitsfor the current cylinder and uses them for the PI saturation using theknock processor 1408 and misfire processor 1418. However, the PIintegrator is reset using the next cylinder's knock and misfire limit.That is, if the output could be saturated by either the knock or misfirefor the next cylinder the integrator will be reset to its correspondingboundary value.

The MBT timing controller of the mixed method 1495 is similar to theaverage approach method (compare FIGS. 38 and 39). Both the average andthe mixed methods use only one PI controller 1440. The difference isthat the average method uses a single knock manager 1450 and a singlemisfire manager 1460, while the mixed method uses multiple ones. Thus,the output timing limit signal 1480 has individual knock and misfirelimits. The advantage of using the mixed method is that only one PIcontroller 1440 is used for all cylinders which reduces the throughputrequirement. Also, use of multiple knock 1450 and misfire managers 1460produces improved fuel economy.

Section H: Closed-Loop Individual Cylinder Air/Fuel Ratio Balancing

This feature of the present invention comprises a method of controllingindividual cylinder air to fuel (A/F) ratios using a closed loop 1300and an ionization signal 100. An individual cylinder ionization signal100 is used to calculate the minimum timing for best torque (MBT) timinginformation of that cylinder. This MBT timing information 1320 is thenused to control the individual cylinder's A/F ratio using a closed loop1300. The control is based upon the relationship between the MBT timinginformation and the A/F ratio. In addition, an adaptive learning methodis employed to modify (or update) the feedforward control logic block ofthe present invention.

The individual cylinder A/F ratio of an internal combustion engine 161varies due to the fact that the intake manifold cannot distributeairflow into the individual cylinders evenly, even when the global A/Fratio (i.e., the average A/F ratio of all the cylinders) is maintainedat stoich. The difference in A/F ratio between individual cylindersaffects engine emission, fuel economy, idle stability, vehicle Noise,Vibration and Handling (NVH), etc.

The closed loop control of an individual cylinder's A/F ratio of thepresent invention utilizes the MBT criterion, see Section F: RobustMulti-Criteria MBT Timing Estimation Using Ionization Signal, providedby the ionization signal 100 or by the in-cylinder pressure signal, seeSection J: The Determination of MBT Timing Through the Net PressureAcceleration of the Combustion Process, to balance the A/F ratios of theindividual cylinders.

This invention is a subsystem of an ignition diagnostics and feedbackcontrol system using ionization current feedback illustrated in FIG. 13.It is labeled 1300 in FIG. 13. When an engine 161 is operated near toits MBT spark timing, it is known in the art that engine MBT timingcriterion, calculated from either in-cylinder pressure or from anionization signal 100, is a function of the A/F ratio at which theengine 161 is operated at. When the A/F ratio increases or moves to thelean direction (i.e., a leaner A/F ratio), the MBT spark timing isadvanced and moves forward from Top Dead Center (TDC). This movement isdue to the fact that leaner the A/F ratio is, the longer it takes forthe combustion flame to develop. FIG. 40 shows a test relationship curveof MBT spark timing versus A/F ratio using a 2.0 L, four cylinder enginerunning at 3000 RPM with Wide Open Throttle (WOT).

When the engine 161 is operated near the MBT spark timing, therelationship between A/F ratio and MBT spark timing information orcriterion (obtained using either the ionization signal 100 or thein-cylinder pressure signal) also holds at the individual cylinderlevel. For the same reasons discussed above, the MBT spark timing of arelatively lean cylinder (i.e., a cylinder operated at a lean A/F ratio)is advanced when compared with cylinders operated with relatively richA/F ratios.

FIG. 41 shows a test relationship of MBT timing information and A/Fratio for individual cylinders of a 2.0 L, four cylinder engine runningat 1500 RPM, 2.62 Bar Brake Mean Effective Pressure BMEP with 20% enginegas re-circulation (EGR) and ignition timing at 47° Before Top DeadCenter BTDC. The engine 161 was run very close to stoich with an A/Fratio of 14.54. Furthermore, the relatively leanest cylinder (e.g.,cylinder #4 with an A/F ratio of 14.96) had its MBT timing criterion (arelative criterion indicating how far the current spark timing of thecylinder is from MBT timing) 2 degrees more advanced than the mean MBTcriteria. Similarly, the cylinder with the richest A/F ratio (cylinder#3 with an A/F ratio of 14.13) had its MBT timing criterion 1 degreebehind (or delayed when compared to) the mean MBT spark timing.

FIG. 41 has been redrawn in FIG. 42 to show the individual cylinderrelationship of A/F ratio and MBT timing criteria. From FIG. 42, it isseen that the relationship of A/F ratio versus MBT criteria is generallylinear even though the data is collected from individual cylinders. FromFIG. 42, it can be determined that a predominantly linear relationshipexists between MBT timing information and A/F ratio even at theindividual cylinder level when the engine is operated near its MBTtiming.

The present invention uses this relationship to balance the A/F ratiofor individual cylinders. The method used in the present invention usesa closed loop controller to adjust (or trim) the fuel of individualcylinders such that all cylinders have the same MBT timing criterion.Using the relationships illustrated in FIGS. 40 and 42, the A/F ratiosof the individual cylinders are balanced. FIG. 43 illustrates the closedloop control method of the present invention for balancing individualcylinder A/F ratios.

This control method consists of seven major logic blocks or steps: a)calculating a mean MBT timing coefficient 1320, b) calculating error ofunbalancing 1330, c) error integration of the individual cylinderdifference 1340, d) feedforward fuel trim coefficient for eachindividual cylinder 1350, e) Rescale trim coefficient for eachindividual cylinder 1360, f) adaptive updating feedforward fuel trimcoefficient 1370, and g) individual cylinder final fueling coefficientcalculation 1380. The control method of the present invention balancesthe A/F ratio between the individual cylinders caused byengine-to-engine variations, uneven intake airflow due to intakemanifold geometry, and other related factors. This controller isdisabled when the engine is either knock and misfire limited. In apreferred embodiment, it is run once every engine cycle so that the MBTtiming information is updated for each cylinder.

The inputs to the present control method are the MBT timing criteriaobtained using the method described in Section F: Robust Multi-CriteriaMBT Timing Estimation Using Ionization. The output from the closed loopcontrol 1300 in the present invention is used as a multiplier of anindividual cylinder fuel command to correct the individual cylinder A/Fratios. The following is a description of each of the seven functionalblocks or steps or logic blocks of the closed loop 1300 of the presentmethod and apparatus (see FIG. 44).

First, a mean MBT timing coefficient is calculated 1320. The output ofthe MBT timing estimation method from either the in-cylinder pressuremethod or an estimate using an ionization signal 100 can be representedas a vector with a size equal to the number of cylinders measured inunits of Degree After Top Dead Center (DATDC). Let L_(MBT)(i) representthe MBT timing criterion obtained from the MBT timing estimation inSection F, where index i represents the cylinder number. The mean of theMBT timing criterion for all cylinders can be calculated using thefollowing formula:L _(MBT−MEAN)=1/nΣL _(MBT)(i),  (Equation 1)1320,where n is the number of cylinders and L_(MBT)(i) is summed from 1 to n.

Next, an error of unbalancing is calculated 1330. The error in the MBTtiming coefficient, i.e., the MBT timing coefficient errorL_(MBTERR)(i), caused by the unbalancing of the individual cylinders iscalculated by subtracting the mean of the MBT timing coefficientL_(MBTMEAN) from the MBT timing criterion L_(MBT)(i) as illustrated inthe following equation:L _(MBTERR)(i)=L _(MBT)(i)−L _(MBT-MEAN), I=1, 2, . . . , n  (Equation2)1330.

Third, an error integration of the individual cylinder difference 1340is performed. The integration of the MBT timing error, i.e., the MBTtiming coefficient integration error for an individual cylinder,ERR_(MBT)(k+1), can be calculated using the following equation:ERR _(MBT)(k+1)=K ₁ * [ERR _(MBT)(k)+L _(MBTERR)(k)],  (Equation 3)1340,where k is a time step index representing the k^(th) engine cycle,L_(MBTERR)(k) is the error vector obtained from step b at the k^(th)engine cycle, K₁ is the integration gain coefficient with a typicalvalue of 0.001 and can be used as a calibration coefficient for theclosed loop control method of the present invention.

Fourth, a feedforward fuel trim coefficient for each individual cylinder1350 is calculated. The feedforward fuel trim coefficient vectorFT_(FDD) (each element represents a corresponding individual cylinder)is the output of a look-up table 1352. It is a function of engine speedand load. Due to the intake manifold geometry, the individual cylinderunbalancing changes as the airflow rate changes. The look-up table 1352is used to compensate for this variation. The combined fuel trimcoefficient is called the raw fuel trim coefficient FT_(RAW) and iscalculated by adding the integration of the MBT timing error, ERR_(MBT),and the feedforward fuel trim coefficient, FT_(FDD) 1350. See Equation 4below:FT _(RAW) =ERR _(MBT) +FT _(FDD),  (Equation 4)1350.Note that when the engine 161 is operated with abnormal combustionconditions (such as knock, misfire/partial-burn, etc.), the MBT timingcriterion will not be used for an A/F ratio balancing calculation due tounreliable MBT timing estimation 1353 and in this case the integratedvalue will not be updated. Consequently, the raw fuel trim coefficientFT_(RAW) is set to the feedforward fuel trim coefficient FT_(FDD) 1354.

FIG. 45 is an example of the look-up table 1352 which is stored inmemory 112. The memory can be RAM, ROM or one of many other forms ofmemory means. Engine speed is mapped along the vertical axis from 650rpm (e.g., idle) to 6500 rpm (e.g., rated rpm) in 12 increments. Thenormalized engine load is mapped along the horizontal axis from 0 to 1,in 10 increments. Thus, the 12×10 data parameter matrix has valuesstored for the feedforward fuel trim coefficient vector FT_(FDD) foreach combination of engine speed and engine load. Normally, this table1352 is obtained through the engine calibration process.

Fifth, the trim coefficient for each individual cylinder is rescaled1360. The raw fuel trim coefficient FT_(RAW) does not have to meet theconstraint that the summation of vector FT_(RAW) equals the number ofcylinders such that overall fuel flow rate is unchanged. The followingrescaling operation 1360 takes care of this in which the raw fuel trimcoefficient FT_(RAW) is rescaled yielding the rescale trim coefficientFT_(SCALED):FT _(SCALED)(k)=(n*FT _(RAW)(k))/(ΣFT _(RAW)(k)),  (Equation 5)1360,where index k represents the k^(th) cylinder, n is the number ofcylinders and FT_(RAW)(k) is summed from 1 to n. That is, the rescaledtrim coefficient FT_(SCALED) is calculated by multiplying the raw fueltrim coefficient FT_(RAW) by the number of cylinders in the engine 161,and then dividing this total by the sum of all the raw fuel trimcoefficients FT_(RAW) for each cylinder in the engine 1360.

With the help of this step, the fuel flow for a given engine cycle willbe the same as the commanded one. However, it is redistributed so as tobalance the individual cylinders. To ensure a failsafe operation,saturations (i.e., upper and lower bounds) are applied to the fuel trimsfor individual cylinders 1362. Let FT_(UP) and FT_(LOW) represent theupper and lower bound vectors for a fuel trim vector. Both FT_(UP) andFT_(LOW) are calibration coefficients. Normally, the upper and lowersaturation vectors FT_(UP) and FT_(LOW) are set in such a way that thereis enough freedom to balance the A/F ratio for all cylinders withreasonable variation range. Typical values of FT_(UP) and FT_(LOW) are0.9 and 1.1 which can compensate a 10 percent A/F ratio variation. Ifany element of FT_(SCALED) is outside of the upper or the lower bound,it will be reset to its boundary value, and the associated unsaturatedelements will be rescaled 1364, using the process similar to (Equation5) so that the mean of the trim vector is equal to the number ofcylinders in the engine 161. The saturated fuel trim vector is calledthe final trim vector FT_(FINAL).

After the final fuel trim vector FT_(FINAL) is calculated by rescalingthe unsaturated elements of FT_(SCALED) 1364, the individual cylindererror integrator for the (k+1)^(th) engine cycle ERR_(MBT)(k+1) will bereset 1366 to reflect the scaling and saturation operation. It iscalculated by subtracting the feedforward fuel trim coefficient FT_(FDD)from the final fuel trim vector FT_(FINAL) and then dividing this totalby the integration gain coefficient K₁. See equation 6 below:ERR _(MBT)(k+1)=[FT _(FINAL)(k+1)−FT _(FFD)(k+1)]/K ₁,  (Equation6)1366.This resetting of the individual cylinder error integratorERR_(MBT)(k+1) 1366 works to prevent overflow and typical integratorwinding problems.

In the sixth step, the feedforward fuel trim coefficient is adaptivelyupdated 1370. In a preferred embodiment, the adaptive portion of theclosed loop control method modifies or updates the feedforward look-uptable 1352 based upon the current engine operating conditions 1372(engine speed and load). When the engine 161 is operated in aneighborhood of the lookup table mesh point, the adaptive algorithmupdates the new mesh point value FTM byFTM _(NEW)(k)=FTM _(OLD)(k)+ERR _(MBT)(k),where k is the engine cycle number and both FTM_(NEW)(k) andFTM_(OLD)(k) represents the updated and current mesh point value. Thegoal is that when the A/F ratio for all the cylinders are balanced, thefeedforward output provides the final fuel trim coefficient.

In the seventh step, the individual cylinder final fueling coefficientis calculated 1380. The final fueling (FUEL_(FINAL)) command for eachindividual cylinder is calculated by multiplying the commanded fuelingor fueling command (FUEL_(CMD)) and the final fuel trim coefficientFT_(FINAL) of a corresponding cylinder as shown in the followingequation:FUEL_(FINAL)(i)140=FUEL_(CMD) *FT _(FINAL)(i), i=1, 2, . . . ,n.  (Equation 7)1380.In a preferred embodiment, the steps (or instructions) in FIG. 43 arestored in software or firmware 107 located in memory 111 (see FIG. 46which is a logic block diagram of the air to fuel ratio control systemof the present invention). The steps are executed by a controller 121.The memory 111 can be located on the controller 121 or separate from thecontroller 121. The memory 111 can be RAM, ROM or one of many otherforms of memory means. The controller 121 can be a processor, amicroprocessor or one of many other forms of digital or analogprocessing means. In a preferred embodiment, the controller is enginecontrol unit ECU 121.

The ECU 121 receives an ionization signal 100 from an ionizationdetection circuit 10. The ECU 121 executes the instructions 107 storedin memory 111 to determine a desired air to fuel ratio AFR for eachcylinder. It then outputs the desired fuel command 975 to some form offuel control mechanism such as a fuel injector 151 located on the engine161.

Section I: Exhaust Gas Control Using a Spark Plug Ionization Signal

Exhaust gas re-circulation (EGR) is an effective way to reduce NOxemissions in an internal combustion engine 161. The external exhaust gasrecirculation EGR widely used in the prior art is calibrated usingengine mapping points. That is, the desired exhaust gas re-circulationEGR rates used in controlling an engine are mapped to various engineoperating conditions such as load and speed. Note that the amount of EGRaffects the engine emission and also its combustion stability. Tomaximize the NOx emission reduction without fuel economy penalty, it isdesired to have high EGR rate, but on the other hand, too much EGR maydestabilize the engine combustion process. Therefore, in some cases itis desired to have as much EGR as possible with stable combustion. Dueto engine to engine variation, engine aging, and engine operationalenvironmental variation, open-loop calibration of a desired EGR rate isvery conservative. In the present invention, a closed loop controller isused to regulate the external exhaust gas re-circulation EGR to maximizefuel economy and minimize emissions. In another preferred embodiment,the internal exhaust gas re-circulation EGR is controlled by the closedloop 1600.

Exhaust gas re-circulation EGR is used to reduce flame temperature andslow down the combustion process. Because of this, it is not used whenthe engine is operated with a light load or at idle conditions. Exhaustgas re-circulation EGR finds its greatest benefit when used in partialload conditions, where the pumping loss is reduced by a relatively widerthrottle opening. The combustion process also benefits from a widerthrottle opening.

At a wide open throttle, where the pumping loss is at its minimum andthe torque output is the priority, the exhaust gas re-circulation EGR isnot used anymore. As stated earlier, open loop control of exhaust gasre-circulation EGR uses extensive engine calibration efforts to set adesired exhaust gas re-circulation EGR rate at various partial loadconditions. Due to the exhaust gas re-circulation EGR and the sparktiming open loop control, the desired exhaust gas re-circulation EGR isusually too conservative to fully take advantage of the exhaust gasre-circulation EGR fuel economy benefit. In addition, the exhaust gasre-circulation EGR rate 1610 is typically controlled by the exhaust gasre-circulation EGR valve position 1620. As the engine 161 ages, theexhaust gas re-circulation EGR valve and its plumbing are clogged by theexhaust deposits and the true exhaust gas re-circulation EGR deliveredto the cylinders could be changed dramatically.

This feature of the present invention uses the ionization signal 100 andclosed loop control 1600 of the exhaust gas re-circulation EGR toprovide the engine 161 with either the minimum spark timing for besttorque (MBT) timing or knock limited timing to yield the maximum fueleconomy benefits associated with exhaust gas re-circulation EGR.

The purpose of exhaust gas re-circulation EGR is to 1) reduce NOxemission and to 2) improve internal combustion engine 161 fuel economywith a given NOx emission level. The formation of NOx depends on twofactors: First, there must be enough oxygen to oxidize the N₂, and theother is there must be a high enough temperature for the NOx formationreaction to accelerate. When exhaust gas re-circulation EGR is inducedinto the combustion chamber, the exhaust gas will act like an inert gasand absorb the heat from the combustion reaction. As a result, the globegas temperature, the temperature at which combustion takes place, isreduced through the EGR dilution effect. The reduced temperature slowsdown the NO_(x) formation. The suppressive effect of EGR on theformation of NO_(x) is enhanced as the volume of the re-circulatedexhaust gas relative to the volume of fresh air admitted into the engineis increased, i.e., the EGR rate is increased.

When exhaust gas re-circulation EGR is used in the engine 161, lessfresh air enters the combustion chamber due to hot exhaust gas taking upmore volume in the chamber. Thus, the air/fuel mixture becomes dilutedbecause there is less room for oxygen in the cylinder (i.e., thedilution effect). In order to meet the same load requirement, thethrottle opening has to be increased to compensate the increase ofintake manifold pressure due to the exhaust gas re-circulation, andtherefore to maintain the same air flow (or oxygen). The increasedthrottle opening not only reduces the pumping loss, but also acceleratesthe combustion process due to stronger turbulence resulting from theincrement of the intake manifold pressure.

Although the addition of exhaust gas re-circulation EGR reduces thecombustion speed because of the dilution effect in the cylinder, thehigher turbulence balances it out to an extent. The result of the twoeffects is that the exhaust gas re-circulation EGR dilution slows downthe combustion gradually as the EGR rate 1610 increases. At the pointwhen too much exhaust gas re-circulation EGR is added to the combustionchamber, the combustion becomes unstable. Thus, the employment of highEGR rates tends to cause instability of the engine operation.Consequently, the EGR rate should be controlled to maintain a balancebetween suppression of NOx emission and engine combustion stability.

One of the measures of combustion stability is the COVariance (COV) ofIndicated Mean Effective Pressure (IMEP) since it increases as thecombustion goes unstable. In order to have the best fuel economypossible, the control strategy is to add as much exhaust gasre-circulation EGR to the combustion chamber as possible withoutdeteriorating the combustion quality. In the prior art, the calculationof the IMEP COV uses an in-cylinder pressure signal. However, it isdifficult for production engines to measure combustion stability due tolack of in-cylinder pressure sensors which are production ready, have alow price, and are reliable. This invention proposes to use theionization signal 100 to generate a combustion stability criterion anduse this criterion to maximize the exhaust gas re-circulation EGR rate1610 and thereby maximize fuel economy at a given emission allowance.

As the exhaust gas re-circulation EGR rate 1610 increases, thecombustion uses earlier spark timing to accommodate the longercombustion duration for the best fuel economy, or in other words, to runthe engine at its Minimum timing for the Best Torque (MBT). When thespark timing is not knock-limited, the minimum time for the best torqueMBT spark timing increases as the exhaust gas re-circulation EGR rate1610 goes up. Meanwhile, the combustion instability gradually increasesdue to the requirement for a longer burn duration. Typically, when 0–90%of the burn duration takes more than 70 crank angle degrees, thecombustion tends to become unstable and is usually no longer acceptable.At this point, fuel consumption starts to increase due to thedeteriorated combustion process. Also, the hydrocarbon (HC) emissiongoes up rapidly due to unburned fuel if more exhaust gas re-circulationEGR is used. This instability limit is reached when the minimum time forbest torque MBT timing is advanced beyond a certain crank degrees BeforeTop Dead Center (BTDC) with a typical value of 40 degrees. To preventthis from happening, the engine exhaust gas re-circulation EGR rate 1610is calibrated so that the minimum time for best torque MBT timing isless than a calibratable value such as 40 degrees BTDC. A disadvantageof this approach is that the calibration is very conservative due to thefact that the actual exhaust gas re-circulation EGR rate 1610 varies dueto engine aging, etc.

The proposed closed loop maximum exhaust gas re-circulation EGR ratecontroller 1600 utilizes the relationship between the engine minimumtime for best torque MBT timing and COV of IMEP (see FIG. 47).Generally, when the COV of IMEP is used as a combustion instabilityindicator, less than 3% is considered as a good combustion. As shown inFIG. 47, when more exhaust gas re-circulation EGR is added to thecylinders, the COV of IMEP increases as the minimum time for best torqueMBT timing increases. When the COV of IMEP is greater than 3%, the MBTtiming is about 43 degrees BTDC and the exhaust gas re-circulation EGRrate 1610 is about 20%.

In the present invention, the minimum best time for best torque MBTspark timing is used, instead of COV of IMEP, as a measure for themaximum dilution rate (exhaust gas re-circulation EGR rate 1610)control. FIG. 47 illustrates the correlation between COV of IMEP and theminimum time for best torque MBT spark timing for a 2.0 L, 4 cylinderengine running at 1500 RPM with 2.62 Bar BMEP.

When exhaust gas re-circulation EGR is added to the cylinder, theinitial temperature of the unburned mixture increases because of the hotexhaust gas. The unburned mixture is more prone to auto-ignite when theinitial temperature is higher and causes the engine to knock. The engineminimum best time for best torque MBT spark timing might not be knocklimited without the addition of exhaust gas re-circulation EGR. However,as the exhaust gas re-circulation EGR rate increases, the engine sparktiming may become knock limited. Furthermore, if more exhaust gasre-circulation EGR is added, the mixed gas becomes hotter and the knockbecomes more severe. As a result, the spark timing is backed off fromthe MBT timing to avoid the knocking. This leads to bad fuel economy.

On the other side, an increased exhaust gas re-circulation EGR ratereduces the engine pumping loss. Therefore, the preferred or optimalexhaust gas re-circulation rate EGR 1606 for best fuel economy is anexhaust gas re-circulation EGR rate which is a little higher than theknock limited exhaust gas re-circulation rate EGR 1608. See FIG. 48.Since a direct EGR rate measurement is not preferable due to its highcost, the present invention calculates a preferred knock limited sparktiming by using MBT timing criteria generated from an ionization signal100. When the engine is knock limited by exhaust gas re-circulation EGR,the engine is not run at its minimum best time for best torque MBT sparktiming 1612. Instead, the engine is run at a retarded (or delayed) sparktiming which is retard from the MBT spark timing. The amount of delay isreferred to as the “MBT offset” 1614, and is quantified by the MBTtiming criterion (see FIG. 48). The exhaust gas re-circulation rate EGRis reduced to the knock limited spark timing.

In summary, a determination is made as to whether the engine 161 isknock limited (1603) (See FIG. 50). When the exhaust gas re-circulationEGR rate is not knock limited, the engine 161 sets its exhaust gasre-circulation rate EGR 1610 to an optimal exhaust gas re-circulationrate EGR 1606 such that the engine 161 runs at the exhaust gasre-circulation minimum best time for best torque spark timing limit (orEGR MBT timing limit) (1607). When the exhaust gas re-circulation EGRrate is knock limited, the engine 161 sets its exhaust gasre-circulation EGR rate to a knock limited EGR rate 1608 at which theengine 161 runs at a retarded MBT spark timing limit (1609). The engine161 uses a calibratable MBT timing criterion to determine the retardedMBT spark timing limit. The difference between the retarded MBT sparktiming limit and the EGR MBT timing limit is called the MBT offset 1614.

J As part of the ionization feedback control system (see Section C),FIG. 49 shows a logic block diagram of the closed loop EGR rate control1600 that maximizes the dilution rate. The EGR closed loop controller1600 works with the closed loop MBT timing controller 1430, 1490, 1495(see Section I). The controller 1600 has five inputs and seven logicblocks, or logic devices. The controller 1600 output is the EGR valvecommand 1630. A functional description of each of the five inputsignals, engine speed (RPM) 135, engine load 1060, knock limit flag1404, MBT timing signal 1480, and MBT criterion error 1438 is listedbelow:

The current engine speed 135 measured in RPM (Revolutions Per Minute) isthe filtered engine speed representing the steady state engine speed.The engine load information 1060 is calculated as a percentage ofmaximum load, fueling or the Indicated Mean Effective Pressure (IMEP).The knock limit flag 1404 is obtained from the closed loop MBT timingcontroller 1430, 1490, 1495 (See Section G, Closed Loop MBT TimingControl Using Ionization Feedback). The knock limit manager 1450 senseswhen the engine 161 is operated in knock limit mode when either theknock flag 1404 and the knock intensity 1402 are in the knock or thenon-audible knock state. The MBT timing input signal 1480 is alsoobtained from the closed loop MBT timing controller 1430, 1490, 1495(see Section G). When the absolute value of the MBT criterion 1435 error1438 is less than a calibrated and ajustable value, the current ignitiontiming is considered at the MBT timing. The MBT timing used in thiscontroller 1600 is a filtered one. The MBT criterion error 1438 is thecontroller error of the closed loop MBT timing controller 1430, 1490,1495. Individual cylinder MBT timing criterion 1435 is calculated froman ionization signal 100 or in-cylinder pressure signal generated usinga parameter estimation method (see Sections F, J). This parameterdiscloses whether the current engine spark is before or behind the MBTspark timing 1612 for that individual cylinder. The criterion error 1438is filtered to remove the combustion-to-combustion variation factor.

The functionality of each of the logic blocks (or logic devices), theEGR MBT limit table 1640, the proportional and integral (PI) gain anderror generator 1650, the proportional and integral (PI) controller1660, the feedforward EGR rate table 1670, the adaptive learning EGRrate adapter 1680, the saturation manager 1690, and the EGR valvemetering controller 1695 is discussed below:

The EGR MBT limit table 1640 is a logic block that functions as alook-up table which uses engine speed 135 and engine load 1060 as itsinputs. It can be stored in RAM memory, ROM memory, tape, a CD, or anyof a number of digital or analog memory storage devices. Each individualpoint on the look-up table can be calibrated by mapping the EGR rate1610 to a specific engine speed 135 and load 1060 condition (see step1700 in FIG. 51 a). It provides (or outputs) to the PI gain and errorgenerator 1650 a recommended EGR MBT timing limit signal 1642 which iscorrelated to a combustion stability criterion such as COV of IMEP(1710). With a desired combustion stability criterion (such as COV ofIMEP is less than 2.5%), a MBT timing limit can be determined.

In a preferred embodiment, the PI gain and error generator 1650 can be aprocessor, microprocessor or any form of processing means. This logicdevice can operate in two different engine states, knock limited or notknock limited. The knock limit flag input 1404 is used to determinewhich of the two states that the PI gain and error generator operates at(1720), a) the knock limited state, or b) the not knock limited state.The PI gain and error generator 1650 output signal 1652 comprises bothof the PI controller 1660 gains (proportional and integral) PI_gain andthe PI controller 1660 input error PI_error. The proportional gain isset to zero at all times.

In the knock limited state, the PI gain (PI_gain) and error (PI_error)are generated using the MBT criterion error 1438 input signal. The firststep is to determine if the MBT criterion error 1438 is less than acalibratable retard MBT timing limit (1730) such as 3 crank degrees. Ifthe criterion error 1438 is less than a calibratable retard MBT timinglimit, then the spark timing is below the offset from MBT timing.Consequently, the exhaust re-circulation rate EGR can be increased untilit reaches the knock limited EGR 1608 (see FIG. 48).

Thus, if the MBT criterion error 1438 is less than a calibratable retardMBT timing limit (that is, a higher EGR is needed), the PI error(PI_error) is set to one and the integral gain of PI_gain is set to acalibratable value (1732) such as 0.1 The result is that the PI controloutput 1662 increases to increase the EGR.

If the MBT criterion error 1438 is greater than or equal to a calibratedand adjustable retard MBT timing limit, then the spark timing is abovethe offset from MBT timing. The exhaust re-circulation rate EGR shouldbe reduced until the spark timing is at the offset MBT timing, or EGRMBT timing. Consequently, the exhaust re-circulation rate EGR can bereduced until it reaches the knock limited EGR 1608 (see FIG. 48).

Thus, if the MBT criterion error 1438 is greater than a calibratableretard MBT timing limit (that is, a lower EGR is needed), the PI_erroris set to negative one and the integral gain of PI gain is set to acalibrated and adjustable value (1734). The result is that the PIcontrol output 1662 decreases to reduce the EGR rate 1610.

In the not knock limited state, the PI gain and error output signal 1652is generated using the MBT ignition timing signal 1480 from the MBTtiming controller 1440, 1490, 1495. The first step is to determine ifthe MBT ignition timing signal 480 is less than the EGR MBT spark limit(1735). If the MBT timing is less than the EGR MBT spark limit, PI_erroris set to one and the integral gain of PI_gain is set to a calibratablecalibrated and adjustable value. As a result, the PI control output 1662increases to increase the EGR rate 1610 until it reaches an optimalexhaust gas re-circulation EGR rate 1606. If the MBT ignition timingsignal 1480 is greater than or equal to the EGR MBT limit, PI_error isset to negative one and the integral gain of PI_gain is set to acalibrated and adjustable value (1739). As a result, the PI controloutput 1662 decreases the EGR rate 1610 until it reaches the optimalexhaust gas re-circulation EGR rate 1606.

In a preferred embodiment, the proportional and integral (PI) controller1660 can be a controller, a processor, microprocessor or any form ofcontroller or processing means. As stated above, the PI controller 1660receives as an input the PI gain and error generator 1650 output signal1652 which comprises both of the PI controller 1660 gains (proportionaland integral) PI_gain and the PI controller 1660 input error PI_error(1740). The PI controller 1660 output 1662 is the sum of theproportional and the integral control outputs. The proportional controloutput is calculated by taking the product of the PI_error and theproportional gain (1742). The integral control output is calculated bytaking the product of the integral gain and the integration of the PIerror (1744).

A novel feature of this controller is that the integrated value will bereset if the combined output (feedforward and PI outputs) is saturated.When this condition occurs, the PI integrated value is set to a valuesuch that the combined output equals the saturated value (see below) toavoid overflow and rewinding.

The feedforward EGR rate table 1670 is a function of engine speed 135and load 1060. This table 1670 can be initially obtained by mapping theengine maximum EGR rate 1610 with a satisfactory combustion stabilitycriterion (such as COV of IMEP) 1750 (see FIG. 51 b). The accuracy ofthis mapping process can be corrected by the adaptive learning processcontroller 1680 (see below). It can be stored in RAM memory, ROM memory,tape, a CD, or any of a number of digital or analog memory storagedevices. The table 1670 outputs a feedforward EGR rate 1672 (1752) whichis added to the output 1662 of the PI controller 1660 by summer 1663 toproduce a desired EGR rate signal 1664 (1754).

As mentioned above, the adaptive learning EGR rate adapter 1680 comparesthe final desired EGR rate 1664 with a default EGR rate generated fromthe current engine 161 operating conditions (i.e., given engine speed135 and load 1060) which serve as inputs to the adaptive learning device1680 (1756). (Thus, in one preferred embodiment, the adaptive learningapparatus 1680 comprises a processor and a comparator). It will generatea correction value signal 1682 for the feedforward table 1670 (1758). Ifthe engine 161 runs at the current operational condition for acalibrated and adjustable period, the updated value 1682 at thatoperating condition is sent to the feedforward table 1670 to adaptivelycorrect values in the table 1670. The adaptive learning apparatus 1680together with the Feedforward EGR rate table 1670 constitute thefeedback portion of the loop 1600.

The saturation manager 1690 is a logic device which imposes an upper anda lower bound for the maximum desired EGR rate 1610 that is allowed. Theintegral output of PI controller output 1662 will be reset if thecombined output signal 1664 (feedforward 1672 and PI outputs 1662) issaturated (1760), i.e., the desired exhaust gas re-circulation EGR rateexceeds either the upper or lower bounds. When this condition occurs,the PI integrated value is set to a value such that the combined outputequals the saturated value 1692. The lower limit is normally zero, andthe upper limit is dependant upon a number of factors such as EGR valvemaximum opening, exhaust and intake manifold pressure difference, etc.The upper limit can also be a function of the engine's 161 operatingconditions.

The EGR valve metering controller 1695 converts the desired EGR rate1664 into a desired valve opening 1620 by outputting EGR valve command1630 (1764). Due to the closed loop control, the accuracy required forthis conversion is much less than with conventional open loop EGR ratecontrol.

Section J: The Determination of MBT Timing Through the Net PressureAcceleration of the Combustion Process

The determination of MBT timing (the minimum spark timing for the besttorque) at various engine-operating conditions requires extensivemapping efforts. The existing cylinder pressure sensor based controlschemes developed for MBT timing control, such as peak pressurelocation, 50% mass fractions burned location, and pressure ratiomanagement, are still based on pressure signal observation and stillneed certain calibration efforts.

This invention intends to use the maximum acceleration rate of the netpressure increase resulted from the combustion in the cylinder tocontrol the spark timing. When the maximum acceleration point of the netpressure lines up with the top dead center (TDC), the MBT timing isachieved. The invention will not only simplify the spark timing controlscheme, but also make the MBT timing search much reliable.

The MBT timing is also called the minimum spark timing for the besttorque or the spark timing for the maximum braking torque. Unless thespark timing at a certain engine operating condition is limited by knockor is delayed intentionally for a specific condition, there alwaysexists the best spark timing where the same amount of air/fuel mixturecan yield the maximum work. For an ideal combustion cycle, a combustionprocess happens instantaneously, when the ignition, flame kerneldevelopment, flame propagation all occur at the same time. The TDC isthe location that the ideal combustion occurs. In reality, combustioncannot finish instantaneously. The MBT timing is the result of constantchanging of combustion chamber volume due to piston moving and thenon-idealistic combustion process.

Traditionally, the search of MBT timing is done through spark sweep.Unless requested by operating condition for delayed spark timing, almostevery calibration point needs a spark sweep to see if the engine can beoperated at the MBT timing condition. If not, certain degree of safetymargin is needed for the condition to avoid pre-ignition or knock. Theopen loop spark mapping usually requires tremendous effort to achieve asatisfactory calibration.

In recent years, various close loop spark timing control schemes havebeen proposed based on cylinder pressure measurements or sparkionization sensing. Based on massive testing data observation, it wasfound that the peak pressure usually occurs around 15 degree ATDC at MBTtiming; the 50% mass fraction burned mostly happens between 7 to 9degree ATDC at MBT timing. The algorithm published in prior art SAE2000-01-0932 controls PRM (pressure ratio management) around 0.55 toobtain the MBT timing. Since the criteria are based on observations andmay change at different operating conditions, each algorithm still needscertain extent of calibration effort. It is clear that the combustionprocess has to be matched with the engine cylinder volume change toattain the best torque. However, there is no sound theory to support whypeak pressure has to occur around 15 degree ATDC, or why 50% burned hasto happen around 8 degree ATDC, or why PRM should be around 0.55 for theMBT timing conditions.

A combustion process is not strictly a chemical process. In fact, it isa chemical process as well as a physical process and is usuallydescribed by mass fraction burned versus crank angle. The mass fractionburned not only signifies how much chemical energy is released at eachcrank angle during the combustion, but also how fast the chemical energyis released. It has a characteristic S-shape and changes from zero toone from the beginning to the end of combustion. FIG. 52 shows the massfraction burned and its first and second derivatives. The firstderivation of MFB can be treated as the rate of heat release or thevelocity of the combustion process; while the second derivative can betreated as the acceleration of the combustion process. After the sparkdischarge, the flame kernel starts to form. Once the flame kernelbecomes stable, it develops very fast and the combustion process reachesits maximum acceleration point. Then the rapid burning period starts andreaches its maximum heat release velocity, and then the combustionprocess is slowed down and then attains its maximum deceleration point.Since the combustion cannot complete instantaneously and the chambervolume is constantly changing, where to align these critical pointsversus crank angle may have significant impact on how much useful workcan be accomplished during the combustion process. If we ignite themixture too soon, the pressure increase due to the heat release beforethe TDC would generate a negative work. If we do not ignite the mixtureon time, the heat release process would not be efficient enough toutilize the small volume advantages right at or slightly after the TDC.Therefore, where to ignite the combustion mixture becomes critical thatbest torque can be reached at certain spark timing.

The mass fraction burned is mostly determined by the well-knownRassweiler-Withrow method established in 1938 through pressuremeasurement. It uses the chamber volume at the ignition as a referenceand calculates the net pressure increase at every crank angle for thewhole combustion process, then normalizes the pressure by the maximumpressure increase toward the end of combustion. The procedure ignoresthe heat loss and mixture leakage during the combustion. Each percentageof pressure increase signifies the percentage of mass fraction of fuelburned at the corresponding crank angle.

Instead of using the mass fraction burned, we will use the net pressurechange and its first and second derivatives to represent the distance,velocity and acceleration of the combustion process. The net pressure isderived as follows:

At every crank angle after the ignition, for the pressure P(i+1)compared to previous crank angle P(i), the difference is composed of twoparts. One part of the pressure change due to volume change can be foundthrough the difference between the P(i)*(V(i)/V(I+1))^(1.3)−P(i),assuming the pressure undergoes isentropic compression or expansion.Then the pressure difference resulted from combustion between these twocrank angle is P(i+1)−P(i)*(V(i)/V(i+1))^(1.3). This difference is stillevaluated at volume V(i). If we want to know the net pressure withoutany volume change since the ignition, the difference will again becompare with the volume at the ignition point as if the combustionundergoes constant volume combustion. Then the net pressure changebetween two crank angles is:dP(i)=(P(i+1)−P(i)*(V(i)/V(i+1))^(1.3))*V(i)/V _(ig).

Finally, the net pressure at each crank angle will be:P _(net)(i)=P _(net)(i−1)+dP(i),where P is pressure, V is volume and V_(ig) is the chamber volume at theignition point. After this, the net pressure is found for the wholecombustion process. Its first and second derivatives can be treated asthe velocity and acceleration of the net pressure, which can be alsoused to signify as the velocity and the acceleration of the combustionprocess (Shown in FIG. 53). Once the air/fuel ratio and EGR rate aredetermined, the peak velocity and the peak acceleration of thecombustion process do not change with spark timing very much. The wholecombustion process is like a distance runner who is entering a race.Where to attain his maximum acceleration and where to reach the peakvelocity determine how good the final result is. As we know, the workgenerated before the top dead center (TDC) is wasted to fight with themoving piston and produce heat. However, it is a necessary step for theflame to establish itself for further flame development. The useful workis done after the TDC. From FIGS. 52 and 53, we can see that thecombustion process reaches its maximum acceleration point at arelatively early stage, which indicates that the early flame preparationis finished at this point. If we achieve this maximum acceleration pointbefore TDC, some of the rapid burning period will be wasted before theTDC. If we attain the maximum acceleration point after the TDC, therapid burning period right after the maximum acceleration point willoccur at a bigger cylinder volume that results lower combustionefficiency. Therefore, it is reasonable to start the rapid burningperiod right at the top dead center, which allows that the most usefulwork to be generated most efficiently. In other words, when the sparktiming is advanced to the point where the maximum acceleration pointlining up with the top dead center, we can obtain the most useful workout of the combustion process and we can achieve the MBT timing.

FIG. 54 shows the torque value at different spark timing for 2500 rpm,7.86 bar (114 psi) BMEP and FIG. 55 shows the corresponding net pressureacceleration curves at different spark timing. It is clear from FIG. 54that the MBT timing is at 28 degree BTDC. The peak acceleration pointsshowing in FIG. 55 gradually advance as the spark timing is advanced. At28 degree BTDC, the peak acceleration of pressure is located close tothe TDC.

The tests conducted at various engine operating condition has alsoproved that the maximum acceleration point locating at TDC is where weachieve the MBT timing. This rule applies to the combustion process withone peak heat release curve, such as PFI (port fuel injection) engines,natural gas engines, and GDI (gasoline direct injection) engines withonly one time fuel injection in the cylinder.

While the invention has been disclosed in this patent application byreference to the details of preferred embodiments of the invention, itis to be understood that the disclosure is intended in an illustrativerather than in a limiting sense, as it is contemplated that modificationwill readily occur to those skilled in the art, within the spirit of theinvention and the scope of the appended claims and their equivalents.

1. An air to fuel ratio control system, comprising: an ionizationdetection circuit; a controller operably connected to said ionizationdetection circuit; memory; and software stored in said memory, whereinsaid software comprises instructions which control a cylinder's air tofuel ratio using timing criterion, wherein said timing criterion isminimum timing for best torque timing criterion and wherein saidinstructions which control a cylinder's air to fuel ratio using timingcriterion comprise: calculating a mean timing coefficient; calculating atiming coefficient error; integrating said timing coefficient error;calculating a raw fuel trim coefficient; rescaling said raw fuel trimcoefficient; updating a feedforward look-up table based upon the currentengine operating conditions; and calculating a final fueling command. 2.The air to fuel ratio control system according to claim 1 wherein saidmean timing coefficient is a mean minimum timing for test torque timingcoefficient; and wherein said timing coefficient error is a minimumtiming for best torque timing coefficient error.
 3. The air to fuelratio control system according to claim 1 wherein said software furthercomprises instructions to trim the air to fuel ratios of individualcylinders.
 4. The air to fuel ratio control system according to claim 1,wherein said rescaling said raw fuel trim coefficient further comprisesapplying saturations.
 5. The air to fuel ratio control system accordingto claim 1, wherein said calculating said raw fuel trim coefficientcomprises adding an integration timing coefficient error and afeedforward fuel trim coefficient.
 6. The air to fuel ratio controlsystem according to claim 2, wherein said rescaling said raw fuel trimcoefficient further comprises applying saturations and wherein saidcalculating said raw fuel trim coefficient comprises adding anintegration timing coefficient error and a feedforward fuel trimcoefficient.
 7. The air to fuel ratio control system according to claim3, wherein said instructions trim the air to fuel ratios of individualcylinders once every engine cycle.
 8. The air to fuel ratio controlsystem according to claim 3, wherein said calculating said final fuelingcommand comprises multiplying a fueling command by a final fuel trimcoefficient.
 9. The air to fuel ratio control system according to claim1, further comprising a lookup table operably connected to saidcontroller, wherein feedforward fuel trim coefficients are stored insaid lookup table.